What to wear??

Kawehi
My sister is 12 years old, and is gay short hair, wears boy clothes and passes as a boy in physical apperance. We're having a church event coming up soon and she does not want to wear a dress, she would only have to wear it for 2 hours and that's it. However I respect and love my sister and don't want to make her uncomfortable or force her to wear something she does not want to. I'm struggling with this because what would be a good alternative to a dress that won't make her feel too girly but still be dressy?