How do you know that much about their financial situation? Are you really the sister OP? I have no idea about any of my friends' siblings' financial situations etc!
If you really are a friend, then I think you need to take a step back. It's weird to be this invested in someone else's family issues.
On the face of it, I agree the brother seems selfish-
However, as others have said, you don't know everything about the dynamics of the situation.
One of my aunts moved in with my grandmother (along with my cousin and her partner) for a while between houses. It strained their relationship a huge amount, and I think it was even worse for her partner and my cousin. For a while afterwards, they just both needed a break from each other. Stuff like that can irreparably damage family relationships even when it looks like one has done the other a favour.
For his children, it may be their first experience of death and they may be really struggling with it. Because of this, he may feel having his mother there wouldn't help them much.
If he'd have to ferry the mother too and from Cornwall twice- presumably leaving his wife and children there without a car? I know this sounds like a small effort for his grieving mother, but in a lot of places in Cornwall, being stranded without a car even for 24 hours at a time would just be shit. Obviously most people would cope but maybe his wife and children are being difficult about it and he doesn't have the strength to argue with them?
Maybe he feels emotionally and mentally exhausted right now, and in a few weeks will feel differently?
FWIW, I do accept that the caring burden often falls on women- but in my own extended family, I have also seen sons do a lot. One of my uncles lived with my dad's mum for years when she was a widow with dementia.
If it's not your family, and not impacting on your life, then why get so wound up about it?