My cousin's EX GF.

I don't see anything wrong here. I know some people would have a shit fit but those people tend to be pretty immature and just can't imagine how or why one would keep contact with an ex. For example, my parents are still great friends with my brothers ex but his wife is totally against that. She even tried telling my parents they couldn't be friends with the ex anymore. She has no say in it. They don't talk about the past relationship or involve the new wife in any way so who is she to tell them who they can talk to?!

My point is, you all seem mature enough for this situation. You can't always help who you connect with, even if it's an ex of a friend or family member. If your cousin and this girl broke up amicably, there should be no reason for anyone to get mad about this, unless they are jealous or immature.