My sister says my ex is her best friend, and now she's including his girlfriend?

My ex-husband and I have been apart for years, I’ve been amicable for the sake of the kids. My sister (and her boyfriend) have always been too friendly with him for my comfort. Recently, she’s invited him and his new girlfriend to dinner at her place, invited them to MY family’s picnics/reunions…then she went on a camping trip with them, her fiance, AND my children. I work alot (he doesn’t support the kids financially) and have to miss events and that is her ‘reasoning’ for continuing to do this. Oh, and he’s her best friend. And there’s more to come with her recent engagement. I’ve said over and over, “I’ve had enough, it makes me uncomfortable and it strains all my relationships” (family and him), she says “he’s my best friend, I want him at my wedding too”. How much is enough? what do I do? He’s no help either, he’s done “enough”...he has our friends, his family…does he really need mine? And yes, I’ve told him to go away. So he went to my sister’s surprise birthday party WITH his new girlfriend. It’s pushing me over the edge. Neither of them are bad people, I love them both. But…what the heck?