I think you seem very grounded, and realistic. And previous posters who have suggested, given you know how ill your dad is, asking for personal keepsakes, is a really good idea.
I come at this from 2 places: my h left 3 years ago for OW Who he has now married. I know he's changed his will to benefit her, rather than our 2 children. It's up to him what he does with what he leaves, it's his to leave. I know that. However, my totally unreasonable emotional side says he left in difficult circumstances, he lied, the usual OW story, and we were married for yonks, we built up that money, we worked for it and planned for it so our children should get a look in, not just her (and her children, ultimately). It would be fair to give ours something.
The second place is, he's been disrespectful of the children's needs since leaving, completely obsessed with OW, the children don't get a look in, in terms of his attention, his time etc etc. And I am really angry at what he's done, how he's done it and the damage he's caused, as well as at his lack of awareness of any of that, because he's so in luuurve with OW
So I can imagine, in these circumstances, I'd have a lot of feelings stirred up and rushing around me and that just maybe, some of the anger that I felt over old stuff, over what I see as injustices, might leak out a bit. I am feeling for your mum here, but also for you, because I wonder if her split from your dad isn't resolved in her, and I fear you may be in for a tempestuous time, as she is able to talk to and confide in you.
I think you know your brother, and you know your relationship. I would encourage my children to talk to each other for mutual support, but what I am saying to you, is that you seem lovely, and understanding, and empathic. If I am right, you might need somewhere to get support over the next few months, because I don't thnk your dads passing is going to be easy for you or your mum.
Take care of you, is my message I think.