So many ideas, but here is one of them. A father is a sports agent. Part of grooming his new clients to sign with him to be their agent is to keep them sexually satisfied. While in his early days his attractive wife (not a model though) would satisfy his clients or help in a group. The young athletes don't know it's his wife though. This is purely to support the business and is transactional and is not necessarily a focus for their own sex life. Their 2 daughters become aware of how mom supports dad as an agent. Sadly mom gets sick and cannot fulfill the satisfaction to help sign on new athletes. The business starts to suffer and the father loses athletes to other agents and is having trouble signing new ones. The daughters become aware that the lavish lifestyles they've had is starting to suffer and dad has been living beyond his means so there is little to no savings. Dad asks his oldest daughter, who is married, college educated, and has a career, if she can help fulfill the duties her mom used to prove to help save the business. She agrees reluctantly but keeps it a secret from her husband as this has been a family secret and she doesn't want to share this with him. The daughter meets with her first client a few times and he signs on. She meets with him on occasion to keep him satisfied. At first it's very much a job but the satisfaction sets in. She consults with her mom to talk about how things are progressing. Given the success of signing a big athlete, her dad asks her to do it again and she is happy to help. This evolves as she becomes more and more slutty in secret from her husband. At some point the younger daughter gets involved too and they build a hugely successful sports agency.

Hope someone can turn this into a rich and intriguing story.