If your sister is "greedy and broke" and your dad has even two nickles to rub together, you can pretty much expect that she's going to do everything she can to grab those two nickles and make sure that you don't get even one of them.

To leave your father in the squalid conditions he was in is definitely elder abuse. It is morally wrong. However, in the USA there is no legal responsibility for adult children to care for, or about, an elderly parent. So that's how they get away with doing that. Even more unfortunately, the local authorities are rather slow to intervene in such a situation, where an elder is living in filth or otherwise not competent to manage their daily life. That's because Americans place high value on letting people do what they want. The results of that are sometimes unconscionable.