An unfortunate counterpoint: Bella actively pushed Jill towards MC, she learns from both of them that things are going well, and despite holding out hope that her husband is still out there, she steals MC for herself...
Ummm, ok give me a sec.... I'll find a way to make it different. (Gotta defend Bella.)
Bella & Jill aren't Besties (just good friends).
Bella doesn't have a working husband and is horny as fuck.
Bianca has a faithful husband (as far as she knows at the start anyway).
Bella knows her husband's corpse is in the locked room??
Key differences:
1) Bella does push for Jill and MC to be together, but her own claim is just as valid as Jill's. You might even say that Jill steals him from Bella if you charm both of them and choose Jill. When Bianca meets him their relationship is much further along.
2) Bella fights her emotions and feels guilty as hell. Bianca seems to give in on the spot. Of course since nothing has actually happened this comparison is unfair, but in order for Bianca to get the high ground on Bella she can't fuck the guy until at least January and has to feel even more guilty about it.
3) Bella has been alone for three years. Bianca has only just broken up. This time the high ground can only be gotten by waiting until the MC is on his fourth year before fucking him.
4) Bella is sexier than Jill and Bianca put together and so MC's ass belongs to her if she decides she wants it. Bianca is exactly as sexy as herself and therefore not sexier than herself and Jill put together and thus she has no such right to MC's ass.
5) As his long lost godmother Bella has the divine duty to see to it that MC impregnates the finest pussy in the game that is to say her own. Bianca is not MC's long lost godmother, but even if she were it would just mean that it would be her duty to see to it that MC gets Bella pregnant.