AITA for not letting my sister use my promdress that I paid for?

I (24F) had my prom back in 2017 and my sister (19F) will have hers later this year. My sister and I don't share clothes but she saw my prom dress and asked to use it. Now I don't just have a standard prom dress. I went to a very expensive school with a lot of rich people. My dress was from a great designer and I paid $1500 for it. It's a beautiful red flapper dress from the 20s and it looks like it was made for me. <br><br>My mom and dad wanted to pay for it but even though I was just 17, I decided to pay for it myself as it cost a lot of money. My dad is on a fixed income and he spends most of the money we make on my brother as he has a lot of health issues. <br><br>We also don't have the same size. I was a lot skinnier in high school and my sister is a completely different size. My sister asked me to let her use it and my mom said to give it to her as it's just collecting dust. I said no but my mom and sister kept insisting. I finally said that the dress is too expensive to be used and I said no. <br><br>My sister began to cry. She would need alterations anyway because of the size difference but they both kept insisting and my mom said that she'd pay to have it altered. I said no again and my sister called me an asshole for not helping her out.<br><br>My dad said that I should just give it to her because she'll probably never get to wear anything that expensive otherwise. He also said that it would be a shame to never use it again. I said it's my dress so I decide what happens to it. My dad said that I'm being selfish but I don't think so. I paid for the dress after all. <br><br>My mom and sister think that I'm being an asshole for not letting her use it and my dad is just staying out of it. AITA for not letting my sister use the dress?