My Mom had a stroke. We could tell moms mind wasn't as keen before she did this & the dr said after the stroke she wasn't getting enough oxygen to her brain. There are 8 of us but we don't have a clue nor will she offer to give us copies of statements or receipts proving where her money is going. Do I have the right to ask for proof?
You said there are 8 of us. Moms kids? How did the SIL get herself in this position of trust with mom and do you suspect she will steal from mom?
You can try to get guardianship of mom, but assuming her son and his wife are taking care of her, that's going to be hard to do.
How is it you happen to know mom did that? I would advise you to see an attorney.
It is not at all unusual for the elderly to gift their money to the people who are there for them in the twilight of their lives. Sometimes it happens because of purposeful manipulation. But that is not against the law.