I have many things about books that I want to say 'cause I just finished watching an adaptation, but we'll get on that another time.
This is just for my preference, but I'd rather have the guy(ml) who had slept with HUNDREDS of different women that the fl doesn't know or has a close relation with (as long as he's clean tho), rather than who slept with ONE who the fl has a close relationship with (sister, bsf, cousin, aunt, mother ffs). And if one of that hundreds is in that parenthesis, that's a NO for me too. I won't read the book, especially when they're still close with that person.
Maybe actually it depends, as long as that person is not really a major character. Idk, just came to my mind. Idc though if it's the other way around HAHAHAHA, 'cause I actually don't mind "brother's ex-gf" trope, or "ex-bf's father" (don't come for me), and I love "ex-bf's bsf".