Firstly, I am sorry to hear about your diagnosis and the loss of your mother. That is a lot for both you and your brother to deal with.

I don't have any autistic family members, but I do work with students with autism and I have some traits myself. As I'm sure you know, autistic people differ greatly from one another and his general level of understanding - whether he has an intellectual disability in addition to autism, for example - will also have an impact on how you would speak to him, as would whether he has any co-occuring mental health difficulties, such as depression.

Of course, you know your brother best, but without knowing him (so I may be completely wrong with regard to him), I would say that hyperfocussing on something like that isn't always a bad thing. It is something I tend to do and while overdoing it can be problematic, it reassures me more often than it worries me. Everybody (with the exception of one colleague, who I suspect has ADHD) objects to my doing it, thinking that it's a sign of anxiety or something, but for me, at least, it generally isn't. It's just how I react to pretty much anything.

Of course, your brother may be different here, but I just wanted to say that his response may be different to yours, but it doesn't necessarily mean he is struggling with it more. He may be; again you know him best, but sometimes reactions are just different.