All I can say is that you have to have along hard think about what you believe is wrong and right here. I was rarely over, say the past 3 years or more in the same room with both brother and mother, and have had no face to face contact with him for two years and that was only very briefly.It must hurt to hear your mother siding with your brother especially when he is nasty to you. With my mother it was more a case of siding with what he said when he was with her and siding with what I said when she was with me.It was frustrating and confusing, but looking back, I know it was about her trying to please both people.Also, looking back, she was no longer making rational choices about things. My brother, who saw her much less often, saw her trying her best to prove she was coping well, whereas I was seeing it all.So she managed to convince him, or agree with him, that she didn't need a care worker in to dose the dangerous medications she was taking(often wrongly). My mother thought she was doing fine.She had dementia and couldn't see otherwise.My brother was supporting her and hindering what was needed. Also, I would say that my mother had a tendency to go with anything my brother said, maybe because of he was older than me and maybe also because she had great respect for authoritarian males.I was just "the baby". He had a lot of shout and in her own uncertain mind she responded to that. When I started believing in myself more, seeking advice(including from this forum) I began to do what I knew was the right thing to do...at which point, my brother at least, began to fade into the background. He could not argue with Mum's dementia diagnosis and he couldn't argue with the memory clinic's advice about instating some care calls.
Yours is, most likely, only similar in some respects to my story.However, you need to put aside, just for a while, the hurt your brother(and mother) is causing you and have a think about what she really needs.Is he meeting those needs for your mother, or not? Seek advice and be proactive in what you believe to be the right course of action. This will give you the confidence needed to make changes where changes are needed.