Sibling Drama

How do you deal with a sibling who is desperatley trying to make up for lost time. My brother had never been very close to my mother and in the last five years he has sucessfully pushed my other siblings away from my mother. My mother is 87 and has perioids of demenia. She is easily manipulated by my brother and both talk poorly of the other members of the family (eight of us). There are three of us girls who have always been there for my mother and have been expected to. She has turned on all three of us due to my brother causing divisions and drama between us. The dementia has turned my mother into someone I dont know anymore. The mean statements and the lack of emotional attachement has been really hard to get used to. With the added stress of this sibling consistently causing drama makes it difficult to care for my mother and dodge his craziness along with the craziness of the dementia. He drinks alot of alchohol with her and at the same time he complains to us about the medications she is taking. Things have been missing around her house.