Our expert says:
Apart from the specifics here, I think people should recognize the difference between a child's birthday party, where SHE decides who she wants to invite, and a family party organized by the parents largely for themselves, even if the child's birthday is the excuse for it, to which THEY decide who to invite.
And here it seems as though their kid made a decision ( who knows why ) about not wanting your child, and the parents have managed to quarrel without anyone actually understanding exactly what it's all about.
And as long as you refuse to speak to each other, you;ll never understand what's happening, nor have any chance to sort it out.
If you take the initiative to try to sort this out ( with both parents, not only the SIL ) you'de be the bigger and more adult person, not "the lesser".
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