Awww, it is a complicated situation, but perhaps you can still invite him and everyone to your Mom's party, and have a separate, small cake for your brother's daughter, and make that a secondary part of the event. I am sure everyone will still mostly fuss over your mom, and it would be nice for his daughter to meet everyone when the focus is not on herself, so she need not be as nervous.

How does your mom feel about the situation? How "with it" is she? Some people at 90 are still as sharp as they were at 20, others are more subdued and quieter, and of course, some are suffering the effects of disease that often accompanies old age, and are barely with it at all.

That would be my suggestion. Talk it over with your mom first.

Then you can send out a mass email, saying "crisis over, here's the new plan" whatever it may be. Any memories people talk about with your mom, after all, will be family history your brother's daughter can absorb as well.