@Danholm,
Your girlfriend needs to go do a course in confidence building.
Weddings are Bride orientated? You would be supporting her not your Cousin?
Family is family, they are with you for life, as for jealous girlfriends not so.
What surprises me here, is this is of a photograph 1 year ago and you state that you and your Cousin are close. If you are close and you've been with your girlfriend for 12 months, thereabouts, then surely you (4) have spent some time together and if not why not?
You don't judge someone via a photo and especially when they never sent it to start with. You don't judge full stop but irrespective you do take the time to get to know someone, especially if they are "family" of the person you are dating and supposedly love.
Family is family. She needs to chill. Believe in herself and your relationship together. It was a photo.