AITA for refusing to fund my sister’s dream wedding after she banned my boyfriend over a high school grudge?

The Story: Family Politics vs. Financial Boundaries

My (28F) younger sister, “Lily” (25F), is getting married next summer. She’s always dreamed of a lavish wedding, and our parents, while supportive, can’t foot the entire bill. Since I’ve been fortunate in my career and we have always been close, I initially offered to contribute a significant amount (around $10,000) to help make her dream day happen.

The problem started last week when Lily sent out the “Save the Dates.” My long-term boyfriend of four years, “Mark” (29M), didn’t get one. Mark and I live together; he is, for all intents and purposes, my life partner. When I calmly asked Lily if there was a mistake, she got immediately defensive and finally admitted that Mark is banned from the wedding.

Her reasoning? A petty grudge from high school. Apparently, during Mark’s senior year (when Lily was a freshman), he was friends with a group that pulled a mild, anonymous prank on Lily’s friend group. It wasn’t malicious or targeted specifically at her, just typical, immature high school stuff from over a decade ago. Mark barely even remembers it. Lily, however, has held onto this resentment for years.

I was stunned. Mark is a wonderful, kind man who makes me incredibly happy. Our family, including our parents, loves him. He has been nothing but supportive of Lily and her fiancé. I told Lily that excluding him based on a decade-old high school issue is ridiculous and deeply disrespectful to me. I explained that we are a package deal for major family events.

She threw a tantrum, accusing me of “ruining her special day” and stating it’s “her wedding, her choice” who attends.

I told her she’s absolutely right—it is her choice who she invites. But it’s my choice how I spend my money. I calmly informed her that, under these circumstances, I was withdrawing my offer to contribute the $10,000.

Now my parents are calling me, claiming I’m being “manipulative” and “holding the wedding hostage.” They argue Lily is devastated and that I should just “be the bigger person” for the sake of family peace and fund the wedding anyway. Lily is currently refusing to speak to me.

I feel awful that she’s upset, but I also feel like I have to stand up for my relationship. If my partner isn’t welcome, why should I be expected to pay for the party?

AITA?