My whole family is shady and I do not trust them. How can I protect my mom from getting used again?

Okay, here it goes. First off, my mom is very manipulating and can get physically and verbally abusive at times, so it isn't easy dealing with her but I do because she is my mother. My mom is a 74-year-old diabetic and has heart problems. She can do a lot for herself but chooses not to. She can also be so naive.

So this is where I need advice. My older sister (which, by the way, neither she nor her kids bother with Mom... unless they want something, that is!) and her husband stole $20,000 from my mom like 10 years ago, said they would pay it back and told her not to mention it to me or my other siblings who don't bother with Mom either.

Okay, so they got away with that one so they sold my mom a house that they knew she couldn't afford. Plus they are licensed real estate agents and made money off my mom. Oh, it gets better. Then they got into her bank card and would pay not only my mother's one bill but theirs as well. No, there is no power of attorney or conservatorship. My mom allowed my sister to pay something only once!

So now my niece, who also doesn't bother with my mom unless money is involved, is suddenly a caregiver. She is hinting at taking care of my mom, but I don't trust her or my sister. They can never help anyone out without getting paid. The whole family is shady. How can I protect my mom from getting used again?