My mother is failing and my sister and her kids are stealing from her. Any advice?

If you are going to make accusations against someone, you will need to have solid proof. Does your sister have access to your mother’s bank accounts or any credit cards of her’s? Do you? If you don’t have access to her accounts or any credit cards and Sister does, it’s going to be very difficult to prove she’s stealing from your mother. Are you your mother’s POA? Is Sister? Are things missing from her home that they may have stolen and sold? You will still need proof. You will need to prove that they stole them and not that your mother may have given things to them. You will need to prove that your mother is not able to make good decisions which may mean having her formally diagnosed by a doctor with dementia/Alzheimer’s. APS will have to document signs of abuse or neglect. If they come out and see no signs of this and Mom says everything is ok, they won’t be able to do anything. It will still be up to you to prove that your sister is taking advantage of an impaired Senior.

Horrible. 

I agree with JoAnn.  Found this, too: Elder Abuse Hotline (800) 677-1116

Also, take pictures of everything she owns every time you're there to document.  If anyone sees you doing that (even mom), say it's for insurance.  You could also say you're filing police reports since she's been robbed several times!  And you should report all thefts to police. 

You said mom's mind is not sharp.  Sounds like you need to quickly get them out, trusted care in, and then get mom moved to a safe home, yours or AL that transitions.

Call Adult Protection Services and ask for the situation to be evaluated.