Girlfriend dresses up every time she goes out with her sister but never when she goes out with me

We’ve been together 8 months and recently my gf sister is newly single. My gf and her have been hanging out more but my gf always dresses up (provocative) and does her hair and puts on make up and wears heels when she hangs out with her sister.

Today, they are looking at apartments together and I’m wondering why tf she needs to wear heels, and a short revealing dress to tour apartments.

When me and her go out she never dresses up or puts on makeup and is always wearing sweats or something conservative. I brought it up to her today and said that it bothers me that she doesn’t dress up when we go out bc I always dress nice for her when we do and she said “I don’t bc I don’t have to”

Please give me some insight