I'm not going to wish my sister a happy birthday after going nc
Is there a reason you don't want to wish her a happy birthday?
Yes, she betrayed me and I don't want to spend any time with her.
Just because is related to you don't mean that you have to do anything for them. If you don't want to go to an event with them then you have every right to not go.
She's my sister and I should spend time with her on her birthday.
So you've changed your mind then? I'm glad you are going to see her after all!