Quick question,
If I get invited to my sister's friends birthday party, Is it okay for me not to go?

Its actually my brothers ex girlfriend who hes still friends with that asked all three of us to go, but I was never very fond of her. Its her quincenerra (I am sorry if I spelled that wrong) She only seemed to be nice to me because I was her bfs sister (and other than that she was pretty competitive with me) I had the answer if she was wondering where my brother was or anything else about him (like I told her) ...but I feel that I would be rude if I don't go. .....so, is it rude?