My brother and sister have power of attorney. My mom is in hospice but doing ok. Us kids are watching her 24/7. We share the responsibility. Sometimes she is without any water when I come over after he has sat with her or she'll just be in her chair in her dirty bathrobe and a dirty depends.
My sister always defends him - I'm at a loss for who to talk to or what to do.
Instead of talking to your sister about him, have you spoken to him? Have you told him what you’ve noticed and asked him if he’s not comfortable with what he needs to do? If he can’t do it, he shouldn’t be blamed, especially if he wasn’t on-board with 24/7 care of her. Maybe have him come sit with her, but when he’s there, have a health aide come in.