Maggie, I will say this and hope it is helpful... if not, I will shut up xx I think part of grief is often some unnecessary guilt as well. Our own feelings of what we should have done differently or better.
Nobody knows, though, what might happen in the future. As far as I can see, your brother made the choices to live, and die, as suited him. And your mum will never be heartbroken about it, as she doesn't know. That is not meant to sound harsh, but just with the hope you can be easier on yourself, without the what-ifs xx