Not asking for hate, but possible solutions would be great. I want everyone to be happy.

My fiancé and I are getting married on a cruise next year, at the end of May. We had talked to people for a month before contacting the travel agent and wedding planner. After talking to all of our guests, we found a perfect date, cruise, itinerary, etc. for all of my friends, which is like 10 people, and all of his friends and family to attend. I don't have many friends and don't have a great relationship with my family, so they don't even know I am engaged. Therefore, I wanted to be able to have my most important friends to be able to join. When everyone invited agreed on a date that allowed all of my friends and all of his guests, I was ecstatic. I make all the plans with the two planners and send out electronic instructions on how to book the cruise with the group discount, and that the deposit is refundable to hold the room. If you're not sure you can make it, book the room with the deposit only, then you have until 3 months before the cruise date to pay it off or refund. For most of his family, there is a group chat that I sent this information to. After having sent the info to a good portion of the list of invites, and many people had commented on how cool this plan was and how excited they were, his mom commented that the date no longer works. I asked why. One of my fiancé's younger sisters's high school graduation is the day after the cruise, and she has to go to a mandatory rehearsal that would be during the cruise. If it doesn't get changed, then her graduation would be ruined, which is what his mom said in the group chat.

I tried to see if I could change it, but if I push it just by a week, then it would cost every single person at least $60-100 or more, minimum, and some people may not be able to come if I push it by just a week as my friends are students, work in medicine, or teaching and the price increase could be significant. If I look into booking after summer, that falls into August and beyond, where the prices return to the same or less than what we already have. The issue there is we have the people in college starting school in August and September. Ironically, that same sister who's the issue may not be able to join because she'd be in college a few states away if we push it to September. I can't do anything after the end of September because of my career. Many of my friends wouldn't be able to join at all if it gets moved to the end of September, which is the only other time that works with my schedule and is the same price. Or we can push it out another year, which my fiancé is adamantly against. He is also very upset and says when will us constantly having to bend to my family stop, especially if we bend to them now. Someone will always have a problem; let's not change it.

We tried to talk with his mom about it, but she said that if the sister can't join, then she won't either. If she doesn't come, then the whole family might not come as a result. They have a pack mentality. She also admitted that she didn't realize that the graduation and wedding fell in the same week because she didn't put the date in her calendar until I sent the instructions. She knew about this date for 2 weeks. My fiancé and I are currently paying for all the venue-related expenses, our cruise and plane tickets, and a few friends' cruise and plane tickets. So no one is paying for anything except their cruise room and plane tickets. Which is essentially everyone going on a cruise vacation, with half of a day being a wedding with cake and champagne included.

TLDR: Fiancé's younger sister's high school graduation falls on the same week as our wedding, and his mom gave us an ultimatum to change the date or she can't come. He is very upset that his mom is choosing his sister's high school graduation over her eldest son's wedding. He doesn't want to change the date either because he also picked the date. We discussed it with the family 2 weeks in advance of booking, and everyone approved it. Only after booking did this problem arise.