Got invited to ex's cousin's birthday. Should I go?

When my ex and I were still together, her cousin had invited me to her 18th birthday. I was really close with her family so I said yes. That was a few months ago so I thought it wouldn't matter anymore. Then I got this invitation on the mail earlier this week. I was surprised to be honest. It was a foreign circumstance to me. I've never been friends with exes let alone their families after a break up. I'm wondering if I should go to the birthday or not. The cousin and I talk once in a while. As a matter of fact, I got a text the same day as the invitation saying I should go her cousin be damned. I don't know if it's appropriate. Should I go???