My step-dad married my mom when I was 13. I wouldn't have wanted to walk around barely dressed in front of him, never did. The girl must feel very comfortable and thinks nothing of walking around in a towel. Why can't she just put something on?

Your gf is not trying to make you comfortable, but has put her daughter above you here, even telling you to go in the other room if you don't like it. That's not a good sign for your relationship. She should be telling her to put something on and stop doing it now that you and your 13 year old son are in the house.

Hopefully, when she gets the robe, this will stop.