Excluding one wedding guest to please another can backfire
Unless there is abuse or otherwise clear and cruel mistreatment, you invite both feuding parties and let them sort it out. It's fair and it works.
The unraveling of your guest list makes this point for me: You chose Julia over Sara and yet Sara is the only one among you acting like a true friend -- and I include you and your fiance in that, and Julia, and Sara's well-meaning but misguided allies. You could all use Friendship Summer School.
There's always a new chance to do the right thing, so, yes: Tell Julia that Sara's friends' boycott has problems of its own, but it awakened you to how poor a choice you made in excluding Sara.
Say that you agreed to it because you remain grateful for her friendship and for setting you two up, but that excluding someone is not the way you want to get this part of your life started.
Say you're extending an invitation and an apology to Sara, and tell Julia she's a "wonderful friend" (your words, not mine) and you hope she will still consider coming.
It's going to be messy, but a mess due to courage, inclusion and the retraction of a mistake is better than a mess from caving.
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