I'm not sure if this is the right place to ask, but the sidebar didn't expressly forbid it, so here goes. This story is my white whale. I read it probably 3-4 years ago, most likely on ASSTR.
If I recall correctly one of the opening scenes was of a teenage boy (main character) waking up in bed with his two younger sisters. The entire family was very into incest and fucked often.
Again IIRC the boy babysat quite a few kids, and had relations with all of them. One scene that stuck out to me was where he was at the park with a girl he was babysitting and she sat on his lap on the swing, and proceeded to fuck. Another specific about said scene was that she could "flex" her vaginal muscles.
If you can help me find this story I will be greatly indebted to you. If thisnisnt the right place to ask this, please let me know where I should go. Thank you very much!