I'm sorry she's ridiculous. I have many similar cousins who did the same to me, except they couldn't even be bothered rsvp / have a conversation about it. Their issue was that I was an old maid\whore for living with my boyfriend before we got engaged/married. I wouldn't go, it seems like it would be hard for you and she's not deserving of respect and support she herself is unwilling to give. Our extended fAmily isn't all that close, emotionally or geographically, so it may be easier for me to brush it off than you. Regardless, congratulations to you! Don't let anyone steal your joy