Is this cruel?

My brother is fighting mental issues right now, but he's been having big problems these last few months. We lost our dad to a two year battle with brain cancer in May, he refused to spend any time with dad or help with caring for him. Mum and I did all the caring. My brother asked my mum to do the garden for them, which she did, then he criticised her work and told her she had done it all wrong. She went back the next day to correct it and he was fuming. My mum and I have a good relationship with his wife, we always message each other etc. So when Mum asked to spend more time with their daughter Olivia (my niece), and I put a photo up on Facebook of Olivia without asking, on her birthday. He has now cut all contact. We aren't allowed to see them, even his wife isn't allowed to contact us. (He threatened to cut us off as dad was on his death bed) So today Mum takes all the toys etc she had bought for Olivia to stay at her house, Olivia was so happy to see her, kept saying Grandma play! Olivia even asked my brother to hug Mum and play with them. He just turns round and goes upstairs. We don't think we'll ever see them again. I've no idea why he thinks he was mentally abused he wasn't. He forgets that Mum endured 10 years of hell while dad was having an affair. I just don't get it. I feel like we grew up in two different households. He just won't let go of the past and is blaming everyone for how he is. Oh and he also hates us because we don't have the same intelligence as him, he calls us stupid. Is there a right thing to do? Is he right in what he's doing? I just don't know?