AITA for trying to convince my (28M) girlfriend (26F) that a childhood kiss / family inside joke was an innocent mistake?

Apologies this is my first reddit post.

My girlfriend and I have been dating for 4 years and were seriously considering marriage until we got in a huge argument about two weeks ago. I love her very much and I do not want to let this disagreement end the relationship. Please help me

My family is very close and would often take vacations together / have reunions when growing up. I am especially close with the cousins on my mom's side who lived nearby, one of whom is my age. When we were both around 10 playing truth or dare, I gave her a quick peck. It was an innocent moment as kids and nothing more. In the moment I remember everyone laughing and it has since become a bit of a family inside joke, embarrassing I know. Unfortunately it has become a de facto tradition for my aunt to tease my cousin and I with lighthearted jokes about how we were 'married' as kids, including in front of my girlfriend. At first she would laugh along but after a few family get togethers she told me these jokes made her uncomfortable, saying it is weird that we still hang out and even using the "i" word. Now she has essentially told me if I see this cousin at all she will break up with me.

I love my girlfriend but family is everything to me. I cannot imagine missing weddings, birthdays, vacations, etc. Am I the asshole for thinking she should accept that it was an innocent mistake???