How stressful for you to have heard all that fighting. It's hard to know when to get involved or not, especially knowing he could shut you out. I hope you approaching calmed them down, even though she didn't want to talk to you.
Have you looked on Facebook for any closed support groups you could join? I typed in brain cancer and a group called Grey Nation popped up, among some others. More if you type in brain tumor. They accept patients, family and caregivers. You might find one that is helpful.
Anything affecting the brain, especially cancer, can cause a change in personality but he's still the same person. Sometimes people forget that. Maybe he's afraid of that.
I have a neuro- degenerative disease in my brain and spinal cord. When I first got diagnosed I didn't want to talk about it with even my close friends or family (just one or two friends, and I controlled what was said). It was my way of not falling apart. I'm still very private, but it doesn't depress me as much. No cure, blah blah. I was never in control, though, and I shut out some good people. Not from life, just from helping. And I'm older!
I said that to say, there is fight in your brother. He needs support but may not know how to reach out. Pride, youth, the cancer, and just being overwhelmed. The baby will perk him up. If he has a favorite sports team, or hobby get something for the baby (an outfit, etc).
My husband beat cancer. Treatment is brutal but it's a battle worth fighting! And you're right, he can beat this! Hang in there!