At first I thought she was jealous... But now I think OP is a narcissist.

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At this party your SIL would get a lot of sympathy, attention, and encouragement.

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This woman would not spew such vitriol against you unless provoked. We do not have the whole story here. Especially a 5-year relationship. For her to flip like this?

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You are trying to paint a very specific picture. But you don't realize by telling us the story about family members taking sides, that you must have done something HORRIBLE for the family to not unanimously take your side.

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Because based on your description, you did nothing wrong and she went crazy on you. SO any sane person, like the people on reddit would take your side. But the family is not. But for people to get as upset with you speaks volumes. People don't just randomly get upset for no reason. There is way more going on here than we will ever find out.