[M][F] Spied of my GF’s Younger Sister – Now what?

So a little backstory to this – I’m [25] and my gf is about a year older than me. Her sister just turned 21. For a while her family lived with me while they were moving from one house to a new house.

While they lived with us, we used mainly one master bathroom for everyone to shower in because of its size. I’ve always had somewhat of a feeling for my GF’s sister (I think) but only because she is way out of my league when it comes to us dating or even hooking up. And to be honest – I don’t see ya hooking up at all, maybe messing around a little bit but she doesn’t strike me as that kind of type. She’s a shy, self-conscious book-word that I believe she’s only had sex one or two times in her life (which is sad). Maybe a few more but not a regular. However I catch myself just fantasizing about all the things I would do to her almost daily!