AITA for refusing to switch vacation weeks with my coworker after I had already paid for everything?

I 29F have worked at my company for four years. One of my coworkers is 34M and we've worked together the entire time. We get along well, but we're not close outside of work.

Back in February I requested a week off for September because my sister and I planned a trip together. We both had to match our schedules, and I paid for my flights and hotel months in advance because the prices were much cheaper. My manager approved my vacation the same week I requested it.

Last week my coworker came to me and explained that his family had planned a reunion during the exact week I was scheduled to be away. He said he had forgotten to request the time off and now our manager would only approve one of us because our department would be too short staffed.

He asked if I would switch my vacation to a later week. I told him I couldn't because I had already paid for everything and my sister couldn't change her vacation days. I would lose a significant amount of money if I canceled.

He said family should come first and that his parents had already booked a large cabin for everyone. I told him I understood it was important, but I had planned my trip for months and didn't think it was fair for me to take the financial loss because he forgot to request the time off.

Now a few coworkers have told me I should have been more flexible because my trip was just a vacation while his was a family event. Others have said it isn't my responsibility to fix someone else's mistake.

I do feel bad because I know he'll probably miss the reunion, but I also feel like I did everything I was supposed to do by requesting my vacation early.

AITA for refusing to switch my approved vacation week with my coworker?

TL;DR: I requested and paid for a vacation months in advance. A coworker forgot to request the same week off and asked me to switch so he could attend a family reunion. I refused because I would lose money and disrupt my plans. AITA?