Story has multiple chapters I believe. I remember the mom and son beginning a sexual relationship on his way to college I think. Mom eventually divorces dad cause he’s cheating. Moves in with son and then the son and daughter begin a sexual relationship on daughters birthday cause it’s her wish. They both get pregnant but then mom has a miscarriage (don’t know why they went that route in the story). Daughter and son marry and mom also marries him but it wasn’t legally binding or whatever but she’s happy with it all and they all live in his apartment near the college I believe.