So my GF of one year whom which we have a really good relationship was just invited to her ex bf's sister's wedding. I know there will be lots of booze (which makes my gf an idiot) and of corse her ex bf (who is always flirting with her and disrespecting me any chance he gets), and bonus is the whole ex's family obviously. She says she has moved on from her ex and for the most part yes, but the rare occasions they do talk it is still flirty and annoying to me. She was super close to the sister however and would love to see the wedding. What do I do here?

I've fucked up a few relationships with jealousy and neediness issues. I think the best thing to do is just let her go to the wedding and trust her, telling her outright or signalling your unhappiness about the situation will do you no favours.

I think getting jealous over another guy gives that person a perceived higher value, like you think he's better than you and a threat which is why you're getting jealous. Of course jealousy is often irrational, but imo that's what it can signal to a girl. Just my thoughts on the situation, in the past I have found it very difficult to let things like this lie and when I haven't kept it to myself it's blown up in my face, so my opinion is just let her go and trust her.

Is there any way you could get yourself invited as her plus one? Then befriend the ex and be alpha.