This is a delicate situation to navigate. Outside opinions only know a portion of the facts so that may complicate what we're saying, too. Simply put, to introduce my thought, is that you ought to give some serious consideration to going.
My reason I feel going might be a good idea is because the whole picture includes more than just the two of you. Your decision to stay away would also impact your relationships with your siblings and could quite possibly sour them. You don't have to go with a bubbly, effusive demeanor to make it look like everything was great between the two of you, Just be present for the rest of your family. I think that would serve you the best in the long run. I think so, but I'm not a master of relationships so I don't mind if you tell me I've painted the bigger picture all wrong. A'best as you work this out.
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