It’s a brother sister story but deals with so many other elements. It dives heavily into the sisters past, I believe the brothers past, and just gives you a crap ton of info. Sister is older, brother is a lawyer I think? They’re both somewhat “messed up’ I guess you could say. It has like 40+ chapters but in the late 30’s they sleep together again for the 100th time and she says something along the lines of is this what it’s like to be in love or something?? Brothers name is mark I believe.