AITA for refusing to give up my dream wedding dress because my sister can’t afford it?

My younger sister “Megan” recently got engaged. I’m thrilled for her and her fiancé, but she dropped a massive bombshell at dinner last week, and now our family is split.

For context, I’ve been engaged for two years and am finally having my dream wedding after this spring, at the beginning of summer. Megan got engaged just three months ago but decided she wanted to get married before me because “summer is the perfect season.” Fine, whatever. I wasn’t thrilled, but I figured it’s her choice. The issue started when Megan saw my wedding dress during a fitting. It’s a gorgeous designer gown that I saved up for over two years to buy. Megan immediately started crying and said it was her “dream dress” and asked if I’d let her wear it first. I thought she was joking, but she was dead serious.

I told her no, because it’s my wedding dress, and she could find something similar. She blew up, accusing me of being selfish and said she couldn’t afford a dress anywhere near that nice. She makes less money than me and has a tight wedding budget. She told me it’s unfair that I get to have “everything perfect” while she has to settle. She even got my parents involved, and now they’re pressuring me to “be a good sister” and let her borrow the dress.
My fiancé is on my side, saying Megan is being entitled, but my parents are calling me heartless. Megan said she’ll never forgive me if I “ruin her wedding dreams.” She’s even threatening not to invite me to her wedding unless I “reconsider.”

AITA for refusing to let my sister wear my wedding dress before me? Or is my family crazy?