My mum said she wasn’t going to take sides and refused to acknowledge that my brother was acting wrongly.

My mother then phoned my brother and relayed everything I had said to her and offered comfort to my brother who was upset. My brother then messaged me again to say leave our mother alone and to stop bitching about him to her.

Fuck the pair of them, then. They're not worth your time. You grew up fine and your brother grew up to be a shithead? That's classic scapegoat vs. goldenchild right there. My BF and his sister are in the same situation - he's the scapegoat and she's the entitled cunt cos she was never reprimanded or told 'no' as a child or teenager. Once they hit adulthood that will never change, so don't bother trying to salvage relationships that are only there because you put 100% of the effort in.