This is a bit long and complicated so I will try to keep it brief.
Older brother divorced early in year, he got custody of preteen boys (mother alcoholic, mh issues) older daughter early 20s not his biological child but been dad to her since toddler. I am really close to her. She has no contact with natural father as he was bad man. She has her own home.
Two weeks after wife gone, brother moves new woman in. We are appalled for boys sake but well that's that and leave them to it.
A couple of weeks later my dad is diagnosed with cancer. Distraught (I am super close to parents) so go to see brother, he doesn't want to discuss it. I ask if he is going to break news to daughter, he says that he isn't as he doesn't really want anything to do with her.
For the next 6 months I travel 120 mile trip to see dad two or three times a week. Brother goes maybe 3, 4 times, still has no contact with daughter who is very close to her grandparents, they are genuine friends. Brother does not talk to me in this time.
Dad died about 6 weeks ago. Brother still not talking to me or daughter, at funeral he blanks all of us, rest of family and friends except my mum. Other people notice.
See him for first time this week when he drops something at my house, niece is here and he and partner blank her. She is distraught, I can't say anything because the kids are there and I don't want a row at this time.
There is no rhyme or reason for cutting his daughter out. I am so cut up with my own grief as well as my mum's and families that the situation is genuinely making me feel ill.
My question is this. I really want to tell my mum how he is behaving but she is struggling with the loss of her husband of 50 years. I have no wish to have it out with my brother as he revulses and slightly scares me. He has been quite volitile with me in the past. My mum thinks he is a saint, I don't want to disallusion her and cause a row so do I just keep quiet and support my niece as much as I can?