Siblings come out of woodwork at the end.

Possibly you should contact an attorney, and hopefully he/she will give you advice. I hope your dad is leaving more of the inheritance to you and your brother since you and he did all of the caregiving. It is very distressing when siblings put money before a sibling relationship, and you may be better off without having a sibling relationship with your sister, if that's the kind of person she is. You sound great, but it wasn't fair of your sister not to help you and your brother. I hope it doesn't lead to any legal dispute, but keep all emails, etc. (any written communication between you and your sister) hopefully where she stated that she refused to help out, etc.

The assets will have to be divided as the Will states whether your sibs help or not, your hands are tied in that respect. I would welcome your sister's help, in fact make sure to accept as much help as she is willing to give, it doesn't really matter what her motives are. You have poa, so she shouldn't be able to get at his bank accounts without you noticing, and as for asking for items, it wouldn't hurt to lock up any small valuables since there are probably caregivers coming into the home ;)