Something similar happened in our family, but this was theft. My nephew, over the space of a few months, stole hundreds from his disabled gran. He'd be round lots offering help, but would 'drop' things round the side of the chair, where she kept her cash, and lift some cash. Things came to a head when my wife challenged her mum about the amount of money she was going through (my wife manages her affairs) and as she wasn't going out, we realised cash was going missing.
Talks round the family, and two and two added up. Family members had heard him rooting round when they were looking after gran up stairs.
Needless to say he was challenged, but would not admit to it. In went a web cam, and its stopped. He's not been round in a year. His mum won't tackle him, nor has he offered to pay back - hes had a seasonal job all summer.
It now means most of the family have nothing to do with him, and his mum won't challenge him. PS no dad about as he left them years ago.
Gran hasn't forgiven him, and if there are family get togethers, he isn't invited.