My girlfriend has been staying in my room more and more lately. Recently she told me she doesn’t think I should apply moisturizer on my sister, who sometimes asks me to apply it for her on her back after she showers. It takes like 1 minute but my girlfriend doesn’t think it’s appropriate for me to do it at all.

I asked what she felt was inappropriate about it. She said it’s the fact that she has to lift her t-shirt or open up her towel for me to apply moisturizer on her back. I explained that we’re twins and we’ve seen each other naked a gazillion times, we can 100% change in front of each other no problem. We don’t, of course, but on the times we’ve had to change in the same room we just do.

My girlfriend has one very older brother who stopped living at their parent’s home when she was 6, so I don’t think she has much context on close-in-age siblings.

I tried explaining all of this and she was still. How to make her understand it’s actually not weird?

tl;dr: Girlfriend doesn’t like that my sister asks me to apply moisturizer on her back because she has to lift her shirt or open up her towel. How to get her to understand it’s not as weird as she thinks it is and she’s making it weirder than it is?