How do you approach who pays what bills when a stepparent is involved & my parent is living in her home?

They are married and he lives there, so I don't understand any objection your siblings would have over his paying his share. It really doesn't seem to me that they should have anything to say about it. Your stepmother likely has records of their finances, should there be any question. If they had a prenuptial agreement, she needs to follow that. Much depends on how close to her you are. If you are quite close, you may want to let her know about your brothers and that they could cause some issues when your father dies, just so she knows about it. Good luck. She seems like a pretty good woman.
Carol

I unfortunately know from person experience that an adult child's perception of their parent's financial condition can be WAY off. If your father has been taking out cash advances and has (or has had) significant debt, that strongly suggests that he doesn't have any money in the bank. If that's the case, there may be no wealth for your brothers to worry about preserving. I understand that your brothers didn't want your father to marry his wife. I know how that feels. But, your brothers ought to look at the big picture. Their father doesn't handle his money well. He has (it sounds like) very little money. He has a wife who is 20 years younger and will likely care for their father until his death. This is no small burden. Dementia can take many ugly turns as it progresses. Even if her husband passes away before his care becomes overwhelming, she will pay a high price, in terms of her mental and physical health, to care for their father. Also, if there is no pre-nuptial agreement and his estate passes to his wife, your brothers have no say in what their step-mother does with the income and assets of her spouse. It's their money and if her husband trusts her with it, that's the end of it. It's very hard for adult children to have adult feelings when a divorced or widowed parent remarries.

There is more to this strange situation than meets the eye. As an adult child, I would personally challenge your father's wife's handling of his finances and ask for a complete accounting. Because they are married she may say, "back off' or MYOB, and she has the right to do that. Sooner or later, however, you will become involved.