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I must say I think that is so horrible of him to accept the invite. Maybe he will change his mind about attending when he realises it gives you a chance to talk with his girlfriend. or if he fears it will force you to talk about him to your family.
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Toward the Light - no definitely do not talk to your ex about it - he would love the centrality of knowing that he can still hurt you. Its like gold for people like him. Don't do it.
I know some people don't like Chump Lady but she got me through. This link below and the comments on it describe all the different ways people have handled your problem.
So don't sweat it - if its too hard and sad don't go. There is no right or wrong way - the way that works best for you is the one you should follow.
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Toward the light,
If its your nephew and your family and you want to go I would consider going. As an inlaw he should feel like the odd man out. Let him bring a woman, man, purple alien. If it's too traumatizing then don't go..no need to look at him. Sounds like a real loser for accepting the invite..and your nephew sounds odd inviting an ex.
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No good deed goes unpunished. I have learned that lesson the hard way, a thousand times over.