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I think the best way to act is to give them nothing to react to....and by that I mean treat them politely and respectfully, XXXXX XXXXX as if they are a work acquaintance rather than as family. You know how at work people chat about neutral topics, and don't get too personal? I think that may be your best bet with the in-laws. Treat the vacation like a company picnic...don't do or talk about anything you wouldn't talk about at work.
I know you wish you could be close with them, but with all that's happened with them in the past few months, keeping it light is a much safer path to take.