I'm on my 4th playthrough today and I was trying to pay extra attention to the Bella components, and this is my current suspicion as to what's kinda going on.
I'm guessing Bella and her Husband James were living together and about 3+ years ago James was either watching their child at home and the kid fell in the pool and drowned, or James was driving and got in some sort of car accident that killed their child. I think Bella initially blamed James for the death and shut him out. This is supported at least potentially by the fact that on Bella's book shelves she has a small assortment of children's books like the three little pigs, and a goofy book (there's a few others too). The locked door I believe is her son/daughter's room.
Bella in the pool told MC that she forgave James and wonders if he'll come home, or if what she's doing with MC is adultery and if she should just move on. Jill I'm guessing may have gotten pregnant as a teen and either had it terminated, or lost it. The pamphlet on Jill's dresser is about some sort of support group (or something similar), and i'm guessing that's where Jill and Bella met and are so close. I'm less sure about my conjecture about Jill, but her questions on the date about which date for sex and such might have been because she might have either had sex too young, or been raped potentially? This would make sense why she leans on the fact that her money doesn't bring happiness. I worry a bit about Rusty being a bit rolled up into this, since she questions MC about his thoughts on him.
Again this is all just speculation, but I think there might be some decent clues around this.
(Edit) Upon watching the rest of the date scene with Jill, perhaps instead of her losing a baby or something like that, she was in a support group because she lost her older sister, and Bella now is kind of filling that void.