in thinking of my own family....if I had done what your brother has to my own mother....my older brother would have threatened me with bodily harm...(not that this is the answer) and under the threat of being cut off from every member of my family, I would have listened to him.
my mother always listened to my brother (him being the eldest and most successful child)....my mother coddled me more but thought lesser of me in the long run.
your brother obviously has no respect for your requests to protect your mother...and your mom is in charge of her own affairs.
all you can do is pray that Karma and common sense kick in somewhere. i'm so very sorry that your weekend is filled with drama.