Family member wants $ to help pay for a lavish party I cant afford for Mom. Why should I have to pay?

While our father was struggling for the last several years of his life, my brother refused to help him. I bought him food, got him health insurance when he had none and fell from several strokes, bought him clothes, and got him into several nursing care facilities.
Now, he wants to show off with a fancy bday party for Mom - an idea that I didn't like because I live only on savings; no income - and wants me to help pay the thousands he committed after he made all the decisions and arrangements without me.
I live with Mom part time - fixing everything in the house and cooking for her (and her worthless husband) almost daily. I pay for things in the house with my money.

Am I wrong for feeling that I shouldn't have to cough up money for something I didn't commit to do?