Fiancé wants me to change wedding date so he can go to his cousins graduation

I can’t believe I’m writing this.


Background: fiancé and I got engaged Jan 9 of this year. From the start, I planned on having the wedding between May-August 2022 and had been very vocal about it with everyone we planned on inviting.
We officially decided on trying to secure a date in May ‘22 on May 8th of this year. We discussed the date with our parents, grandparents, and 20 person wedding party. Miraculously, everyone was available on May 7th 22. My grandparents are out of the country for all of June 2022 and my wedding party has commitments on a bunch of diff dates so I didn’t think it was possible. We signed our contract with the venue on May 16th.
Now over a week later, his sister (who I don’t get along with and have no relationship with) told him our wedding date was the same date as his cousins wedding. She is graduating from medical school. After panicking last night, I call this morning just to find out the university has no idea what day graduation will be on, but there is a chance it’ll be that Saturday.
My fiancé’s sister implied that if we don’t move it, other relatives won’t show up. Even though they could logistically go to the graduation ceremony and drive straight to our venue and still make it on time. Apparently that’s less reasonable than me changing my entire date.
Besides the venue, I have 2 DJs booked (long story), photographer, and videographer all booked. I also know for a fact that my venue doesn’t have anything available in May. June is basically out bc of my grandparents. April is rainy and tornado season and really not an ideal month for outdoor ceremonies where I live, and having an outdoor wedding was only 1 of 2 things I really cared about having for my wedding day.
Am I in the wrong to be entirely floored? The audacity. I just can’t believe it. If it was that important, why didn’t his cousin let us know her conflicts anywhere between Jan 9-May 16 BEFORE we signed contracts? Also, she’s not in the wedding party, so I already think it’s ridiculous to change the wedding date because of her. I understand why it sucks but I still don’t see why they can’t just drive back instead of causing me insane amounts of stress and making me spend at least 1-2 days moving my wedding day (IF, I can even move it. The contract states my deposit is non-refundable and non-transferable). I guess because more of them would have gone to the graduation if our wedding wasn’t the same day but they livestream the ceremony so I don’t see the big deal.