My mother-in-law is asking my adult children for money. How can they say no to grandma?

I need help! I just found out that my mother-in-law, who lives with my husband's brother, is manipulating my children into giving her money. She and I have a strained relationship, so I try to let my husband deal with her, but I'm very upset about this. My husband has tried to find out from her if she is having financial issues, but she says she is not. My children are adults, so they can do what they want with their money, but I feel it is a horrible thing for her to do. How can they say no to grandma? She doesn't support herself and gets a SS check monthly. It is well known in the family that she likes to spend money. She doesn't have a job either. She could work because she is still healthy (in her 60's). I am just so upset about this. She is telling my oldest daughter that she needs the money because her son (my husband's brother) is draining her financially in some way. We don't see any evidence of it. Advice?