What can I do if my sister and her children are stealing from my elderly mother?

My sister and her daughter (son too I think) have been stealing from my mother for years. She allowed it to happen because she "didn't want to get them in trouble" Now she is elderly, living on social security and they are still doing it. My brother's and I are trying to put a stop to it, but mom still doesn't want to get them in trouble. They most recently took the money (600.00) she has been saving to pay her taxes. We gave them a deadline of August 3 to pay it back, they gave her some of it and asked for more time. We want her to report it but their latest dirty trick is to tell mom that my sister has brain cancer (she gets cancer every time she is in trouble and my mother believes her every time). Anyway, we are at the end of our rope and not sure who to call for help! Any suggestions would be appreciated.