if i were in your situation, i'd ask him to talk with her. does he have a problem with how she acts? he knows it upsets you, why does he allow it to continue? if they are friends, she should respect the relationship he is in with you.
i've not been in your position--but i have been in your fiance's. i felt it was my responsibility to handle things--yeah, my friend and i went way back, but if it made my partner uncomfy to see us being touchy feely then i felt it should stop. out of respect for him, i spoke to my friend. out of respect for me, my friend listened. if my friend hadn't, i'd have distanced myself from that friendship (granted, this wasn't just some guy i was dating--we were getting married as well).
it's not a matter of him choosing between you--but a matter of him respecting your feelings. you aren't saying they can't be friends. asking him to be clear in establishing boundaries with her (that you and he can both live with) is more than appropriate.
as for the weekend, i wouldn't tell him he couldn't go (you are his woman not his mother). but if you don't want him to go, tell him that. if he decides he's going regardless, deal with it. if he doesn't, be sure to make it a really nice weekend with plenty of twosome time!
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