I have a problem. My girlfriend has a younger sister. We are all in our early 20s. They are very close, live together, and do a lot of things together. I love my girlfriend with all my heart. In fact, one day I plan to marry her. My problem is her sister. My gf and her sister look very similar (people often mistake them for each other). Her sister is also very attractive. Within the last year, she has become "overly friendly" if you know what I mean. She isn't doing anything blatant, but it's obvious to me she coming onto me. For example, when she walks past me, she might brush her chest against me as if she's squeezing through when there is plenty of space. Or she will leave her bedroom door partially open when she is changing so that from where I'm sitting, I can see straight in. I would never cheat on my girlfriend, but I can't help thinking about sex with her. I know the simple answer is to stay away from her. The problem there is that the two girls are so close and live together that I would pretty much have to stop seeing my girlfriend too, and that's not an option. I don't want to tell my gf about this because I don't want to ruin their relationship. I also have a feeling it would open up a can of worms about why I waited so long to tell her and how I felt about it all. I don't want to get into that mess. I really can't confront her sister about it because I know she would just deny it as if I'm crazy and then I'd worry she would tell my gf about the conversation and make it seem like I'm coming onto her. The worst part is much of this stuff happens while my gf is around and she's oblivious to it. Earlier this week, the 3 of us were swimming in the pool. My gf tells me to grab her sister while she tickles her (my gf just loves to tickle her). I didn't really want to for the reasons stated, but my gf would get upset if I didn't because to her, it's as though I'm not on her side (this has happened before). So I just said ok. As her sister starts to squirm to get out, she "accidently" rubs her hand up and down my crotch a few times. It happened pretty quickly, but it was obvious to me what she was doing, so I let go and jumped back. My gf didn't notice this. This seems to happen right under my gf's nose but in a way where she doesn't notice it. I want figure out a way to stop this before it gets out of hand. I don't want to ruin there relationship over this and I definitely don't want to give in.
I need advice on how to delicately handle this situation.
As a side note, I'm a bit sexually frustrated because my girlfriend is kind of grossed out by semen, which really limits what we do in bed. This makes the situation harder for me.