Yeah this is the kind of think i was thinking. Something silly! The thought that this would lead to sleeping with her hasn't actually crossed mind. But the thought of how she would look naked really has! I was thinking that seeing as i know where her room is and where she gets dressed and undressed, maybe try walk in "accidentally" and see how she reacts to it

Not so sure about that approach: that would be you "invading" her privacy space, vs. what loves2spin suggested. Light hearted teasing about an unknown somebody yanking her towel when she is standing in the kitchen or living room is very different than you "innocently" walking in on her when she is in her bedroom... Just sayin'....

Or maybe a sentence like
"Don't you afraid of loosing your towel ? I might see you naked then !"

My advice is to let her dictate what happened. Never initiate and never push too far. You are in an uncharted territory because you don't know what she intended to happen, how your own girlfriend would react, and what you're supposed to do.

Maybe try to get your girlfriend come with you once in a while to see if the sister behaved the same way while your girlfriend is around.

Yeah exactly! If she's willing to walk around in little to no clothing in front of me, I'm more than happy to see it! She has great body and wouldn't want her to start feeling awkward. I'd like her to feel relaxed and she might end up showing more.
In regards to your other point, i have been there a number of time with my gf, and she has behaved differently. It seems most of the time (but not every time) that i go there she will leave to go shower or bath or change. But if I'm there with my gf we'll all just sit and talk, and if she does go she'll come back in her normal clothes

nice
 

interesting, although you should behave the same way, even if its just for show

That is soooooooooooooooooooooo old. I remember reading that in a daily joke email I was subscribed to way back when very few people even knew what email was. (And the joke is actually probably older than that.)

towel
 

Yeah this is the kind of think i was thinking. Something silly! The thought that this would lead to sleeping with her hasn't actually crossed mind. But the thought of how she would look naked really has! I was thinking that seeing as i know where her room is and where she gets dressed and undressed, maybe try walk in "accidentally" and see how she reacts to it

Don't walk in- too aggressive. The next time she walks out in her towel- "Is that the biggest towel you have, or the smallest?". Or- "Don't you have any smaller towels?"

Or- hand her a hand towel or washcloth: "Here, I got you a fresh towel".

All with a big friendly smile, maybe a chuckle.

Look, but don't touch- eventually it'll turn your world inside out, in an unpleasant way. NOT worth it. Good luck!

I disagree so much with this. Walking in can at least be faked into an accident if the situation permits. Making jokes about towel size, or handing her washcloth is straight up making a move on her that can't be faked into anything else.

So I was over there 2 nights ago, and she went for a bath, was considering walking in but had no way that I could pretend it was an accident, plus I've decided i don't think my conscience would let me as it seems a wrong to do that! I wouldn't want to break her trust.
However i waited til she came down, and she was in a towel like she sometimes is so thought I'd test the waters. I made a comment along the lines of "I don't know how you manage to walk around like that, aren't you worried you might drop it?" She laughed and said "I'll be fine, plus it's not like it's anything anyone would want to see!!" and laughed and smiled at me. Then a little while later she said to me jokingly "well of it offends you so much, I'll go get changed then!" But i could tell she was joking by the way she said it so i said "oh don't feel you have to on my behalf" I thought that might be pushing it, but she was definitely smiling and had a little twinkle as she said it!