I'm going to stand up to my girlfriends aunt
What has she done?
She takes forever when she has to return things in the house.
You should respect your gf's family. If you think she is too slow returning things talk to her about it so she is aware, because she might not be.
...
It is a good conversation to have if she is upsetting you. Just make sure to be respectful when it comes time to get your point across. No need to go about it in a rude way.