Hi, lost and exhausted, and welcome to Psych Central! I'm sorry your brother is being this way. He's probably thinking she's coming to the end of her life and he wants "his share and more," even though you have been the caregiver. You won't be able to keep him from running his mouth, but you can look after yourself and your mother. Does anyone have power of attorney (durable and medical)? If not, then it needs to be done ASAP.

Is he trying to get your mother on his side--turning her against you?