Well, for me, I still like Bella and this hasn't changed my opinion of her.
The only way I’d kinda be ok with this (and this is total blind speculation) is if there’s something more to the “why”.
Hypothetically if Bella is Jill’s older sister’s ex-lover. She was in love with Jills sister and her big dark secret is holding a torch for this woman. That room has women’s boots in it... maybe that’s her old room? Jills sister lived with Bella back when it was her parents house (we assumed it was her husbands but as her parents it could easily have been where she lived) and it’s kept as it was after her death.
She befriends Jill (explaining how a 35 year old and a teenager became such good friends) and one drunken night kisses her out of longing for her past love.
Jill wasn’t into it, understood the why... they promise to never mention it again and Jill is aware of fact she triggers these memories of feelings in Bella. So she avoids sleeping in her bed and stuff.
Thats an example. If the “hook up” was just a kiss done for real reasons rather than random sex between two otherwise heterosexual women both of whom have strong opinions on infidelity. I could understand it.
Otherwise what we’ve got is Jill being strongly opposed to infidelity but fine getting with someone’s wife... she’s embarrassed at seeing someone naked but OK with hooking up with 35 year old teacher at the university that bears her name. Bella is conflicted about infidelity but having one night stands with her friends multiple times.
It’s just a theory and one I’m sure has been presented before but if Bella was deeply in love with the sister and when she died she was partially invested in this marriage to James... in a moment of weakness kissed Jill because of her resemblance and nothing else is going on I’d actually like Bella more in that scenario. It’s consistent and she keeps her morality.
if this devolves into yet another VN harem where you start a threesome with girls who sexuality is basically just “will do whatever MC wants me to do” id entirely check out.