Aww, it's too bad your brother ruined the surprise, and your mom doesn't want a party. Maybe you, your sister and niece can go over that day and take her to a nice calm lunch instead, and your brother can do something separately with her, and introduce her to her granddaughter one on one. And that way she gets a spread-out celebration, too! Maybe buy an oversize card, and everyone who visits individually can sign their names to it, so she'll have one big moment?

It's funny, I knew if we had asked, my Dad might have said "no" to his 80th birthday party, so we went ahead and planned it and gave him a heads up that he needed to be home that afternoon ... and I am sure he enjoyed the whole deal despite himself! We also warned him that this is just what happens when one has a zero-year birthday, so be warned!