Your ex-wife doesn't own her family but it is her family of origin and so you have to overcome the coldness and ask her how she'd feel about you coming.
You should get the feelings out in the open first. This conversation may also help settle lingering resentments. You don't want to be blindsided by a misread of the situation or her feelings.
It sounds like there's going to be a lot of people there; you wouldn't be forced to interact. But you should get the feelings out in the open first.
If she's fine with you being there, then enter the gathering with respect and cordiality, making sure to acknowledge your ex-wife's presence.
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