It was the first Saturday in August, and the weather in London was warm. Effie and I had known each other for only a few months. We were visiting her brother Stephen, and we had gone out for a walk in the park near his flat. We were lying on our backs on the grass, my head on her lap, enjoying the sunshine and each other's company, relaxing.

After we had been chatting desultorily about pretty much everything, the conversation had inevitably turned to the things that we enjoyed. We were talking about sex; anal sex.

I have always enjoyed feeling my cock inside a woman's bum. I loved the tightness of it, the intimacy, the feeling that it was a little bit dirty.

But Effie had always said no.

I had once told her, teasing, "You are the first African woman I have ever known who has not had a cock in her bum." It was true enough, but I remember thinking that I was taking my life in my hands by saying such a thing. Fortunately, she hadn't killed me. But I think she had wanted to.

So there we were, my head on her lap, me thinking how much I wanted to fuck her. In her bum, deep. And she was still saying, "No, not my bum."

I rolled over and looked into her face, as I put my hand under her dress and started playing with her naked pussy. She looked at the people walking past us, and her legs opened for me. She liked it that people could see what I was doing.

"Why not, love?" I asked. "You've never really told me. You just say that you do not want me there."

She looked at me. "Do you really want to know?"

"Of course I do."

"Do you think that you can take it, if I tell you?"

Whatever she was going to tell me, it was making her pussy wet, so I answered without thinking. "Yes, love, I am sure I can, whatever it is."

"I wonder if you can," she said quietly to herself.

"What is it, my darling?" I asked.

For a while she said nothing, her face down. Then she raised her face. There was a challenge in her eyes as she looked straight at me, but her pussy was dripping.

"I keep it for my brother."

I felt her pussy tighten as she said it. If she'd been wearing knickers, they would have been soaked.

At first I thought that I had not heard her correctly, even though a part of me knew that I had. I felt my stomach clench.

Effie is fucking her brother. And he fucks her in her bum. She keeps her bum for him!

I didn't know what to think; I didn't even know if I was thinking at all.

How long have they been doing this?

Surely not, I cannot have heard her right. But I know I did. Is Steve really fucking her? In her bum?

I felt her hand squeeze mine, and I looked at her. She seemed concerned. Her eyes looked at me questioningly.

"Um, ok, so you and your brother fuck."

"Yes."

"How long have you been doing it?"

"Four years."

She was now twenty six, so at least she had not been young when they started. Well, not very. But still, four years! They've been fucking each other for the last four years!

She squeezed my hand again. "Are you ok about it?"

"Well, yes, love, I guess so. It's just that I would never have thought..." My voice tailed off.

"There's nothing wrong with it," she said defensively. "We know what we're doing. We like it."

"Yes, love." I didn't know what to say. There were a thousand questions racing through my head. "In your bum?" I found myself asking, stupidly.

"In my bum. He prefers it. And I like it."

"That's nice..." My head was still spinning, but I found that my cock was hard. Very hard.

"That's why I don't let you do it. It's always been my brother who fucks my bum, and it would feel strange if you did it as well. But you have my pussy, love, my pussy is always yours. Whenever you want it, my pussy is there for you. Take it." She sat up and held my face to hers, kissing me tenderly. "My pussy is yours, love."

"Your brother, he fucks your pussy as well?"

"Of course, darling."

So her pussy, which I had somehow never thought of as ever having another cock in it, was also being fucked by her brother. Presumably regularly. I was sharing my girlfriend's pussy with her brother. And he was fucking her bum as well. Preferred it, even.

A thought struck me. "What about his wife?"

"She knows, darling."

"But, do you and your brother..."

"Yes, darling. Of course he fucks me in front of her. Why not?"

Why not, indeed? I didn't know what to think, but I knew that I wanted to fuck her right there, right then, in front of everyone.

"I want you," I said.

"Right now, love?" she asked, sounding pleased. "People will see us."

Instead of answering, I was opening my trousers, and I moved between her legs as she opened them for me. I fucked her, heedless of the people stopping to look at us. One of them even clapped as we came together.

We lay there, tired but happy.

I realised that I liked it that her brother was fucking her, that I was sharing her with her brother. Then I remembered that he was back there in the flat, right now. I would see him as soon as I walked in. What would he see on my face? Did I really like it that they were fucking each other? I wasn't sure, and I felt my stomach tense again.

Effie glanced at me, a look of concern in her eyes. "You ok?" she asked.

"I'm ok." I paused, not sure, and decided to try not to think about it. "It was nice fucking you like that, out here. Did you hear that person clap, when we came together?"

We both laughed.

"I think you were turned on about my brother fucking me," she teased.

"Yes, love, I was. I am, I mean. I need to get used to it, but I like it." But I was thinking that I wasn't sure.

"Wait until we get to the flat," she said with a mischievous grin, getting to her feet.

As we approached the door, I felt tense and slightly nervous.

I was anxious about facing Steve, knowing now that he was fucking Effie. As for her, she seemed to have an air of restrained eagerness. We opened the door and went in.

"Hi, Steve," I called out.

"Andy, how are you doing?" came the cheerful reply.

He was relaxing in a leather armchair in front of the t.v., a can of beer in his hand.

"I'm fine," I said, but I was thinking: this man fucks my girlfriend, who just happens to be his sister. Irrationally, I expected him somehow to look different, but of course he was the same as ever, the same old Steve. Whom I now knew was fucking his sister, regularly, in her bum, and had been doing so for the last four years. In her pussy as well, come to think of it.

A small voice inside my head was saying, "Yes, Steve fucks his sister, but is it really so wrong?" I tried to silence the voice, but couldn't. Then I found myself thinking that I did like it. I liked it, and I felt guilty for liking it.

He looked up, and waved his hand lazily at us. But this time, Effie didn't just say hello to him as usual. She went up to him, put her arms around him as he stood up, and kissed him deeply. When they stopped, he was clearly surprised that she had done it in front of me, and he looked over to see my reaction.

"It's ok," she said to him, "He knows. I told him when we were in the park, and then we fucked."

"He knows? You two fucked in the park?" Steve looked bemused.

"Yes. Somebody clapped when they saw us come. Now fuck me."

She slipped out of her dress and let it fall to the floor. She had no other clothes on. Going to her knees in front of him, she unfastened his trousers and pulled them down, then pulled down his underwear. His cock was already hard for his sister. He stepped out of the clothes.

She took her brother's cock in her mouth and started sucking it. Steve was watching me, his eyes half-closed with pleasure, his hands in her soft, black hair. I saw his sister take his cock down into her throat.

"Four years," I was thinking. "They have been doing this for four years;" and then, foolishly, "I wonder how she learned to swallow his cock like that?"

She stood up and came to me, and kissed me tenderly.

Her brother lay on the white carpet, his legs stretched out towards the leather armchair, a red cushion under his head, waiting for his sister to come and fuck him.

My cock was hard, and I wanted to fuck her. I knew that I also wanted to see Steve fuck her. My moral compass was spinning erratically.

"Do you like seeing us?" she asked me softly.

"It is lovely seeing you with your brother."

She took my hand, pleased. "Come, I want you to sit over here."

She sat me down in the chair by her brother's feet, then went to her brother and licked his cock, leaving it wet. She squatted over him, facing towards me, her back to his face. Placing him against the entrance of her bum, she slowly lowered herself. She sighed with pleasure when she finally had him fully inside her, his balls against her pussy. She reached down to hold his balls, closed her eyes, and started moving gently. They fucked slowly, sensuously, enjoying each other's bodies.

I sat there in the chair, watching Stephen fucking his sister, looking at his cock in her bum. It was the most erotic thing I had ever seen. The little voice in my head was now saying, "See how much they are enjoying it; how can such a thing be wrong?" I thought to myself that the voice was right, and I stopped trying to silence it.

She opened her eyes, and saw me with my trousers open, my cock out, stroking it as I watched her brother fuck her. I knew that I would never forget the sheer pleasure of seeing them for the first time. I felt privileged to be watching them, to share in this intimacy between brother and sister, to see the enjoyment on their faces.

They began to fuck faster, harder, her brother pushing up deep into her bum. She was moaning. She looked wonderful.

She leaned forward, moving onto her hands and knees, her brother now kneeling behind her. He was fucking her hard, roughly. He reached under to her breasts, pulling and twisting her nipples. She was gasping, enjoying the pain, and I could see that she was going to come. She started squirting, and I saw her eyes roll upwards as first she came, and then her brother. She shut her eyes, and slowly relaxed.

I wanted to put my arms around her, hold her, kiss her, feel her against me. But she was with her brother. I didn't want to intrude on their pleasure with each other. I felt part of them; I felt apart from them. I felt a little jealous.

She opened her eyes, her brother's cock still inside her, and she beckoned to me to kneel down in front of her. She took my cock into her mouth and into her throat. I came quickly, and she swallowed all of it.

Her brother stood up, and went to fetch some tissues. Effie turned around, showing me her bum. She parted her cheeks with her hands. I saw her body tense, and her brother's semen started to dribble out. She looked around at me, smiling as she squeezed out more of it, then took some tissues from her brother and cleaned herself.

Later, as we were driving home, she asked me, "Are you ok with it, love? Did you like it?"

"My darling, I do like it, very much. I really do. I am happy that you fuck your brother. In fact, I am even a little jealous of him." I smiled at her.

"Don't be jealous, love. I have my brother, and I have you as well, and I love you." She squeezed my leg. "Next time, if he fucks my pussy I want you to be in my bum."