Dude, think of how many guys would love to be you! Your girlfriend adores you and does anything you ask. (Well if she does absolutely anything for you, that indicates she may be a tad shy with her sense of self. A relationship of equals is ideal. But still, there's always something to work on in a relationship and how comforting that you can work on things together. There must be a lot of goodness and comfort and support with you and her.)
And I ask you to consider that our culture teaches us to never be quite happy or content, no matter how good things are. So it's probably that part of your mind talking to you. Hypothetically, if you could wave a magic wand and have this sister's girlfriend in your life and bed instead of your girlfriend, you would be pleased like a kid in a toy store and that would last for 6 months max. Then you would miss your girlfriend and depression would sink in. And you can't have it both ways. When you go to a restaurant, you can only pick one dinner from the menu. Everything else is stuff you don't get. Which ONE do you want?
I totally disagree with the guest who suggested you leave your girlfriend. That would hurt her deeply. You just need to straighten out your priorities and realize YOU CAN'T HAVE EVERYTHING. And what's so attractive about this sister's girlfriend? Is she pretty? Intelligent? Do you actually KNOW HER? What is her personality is like? What problems would crop up in a relationship? A relationship is about a lot more than hormones you know.
There's a country song entitled Then What? Have you heard it?
Then What
(Randy Sharp/Jon Vezner)
Well I got a good friend who's got a good life
He's got two pretty children and a real nice wife
But he never seems quite satisfied
I said I know what's on your mind
But you better think about it before you cross that line
The grass aint always greener on the other side
Then what, what you gonna do
When the new wears off and the old shines through
And it aint really love and it aint really lust
You aint anybody anyone's gonna trust
Then what, where you gonna turn
When you cant turn back for the bridges you burn
And fate can't wait to kick you in the butt
Then what
Well I aint saying that looking's a crime
Well I've done my share from time to time
It dont mean that you gotta take that leap
When you're standin' on the brink
Before you jump you gotta step back and think
There's price for ever promise you dont keep
And you could do something new with your girlfriend to give a fresh touch to your relationship. For example, stay a weekend at a nice bed and breakfast that you've never been to before. Sit in the hot tub and sip some French wine and tell her you love her and appreciate how caring she is. And ask what SHE is interested in, ask how SHE feels about the relationship, what's good, is there anything she feels uncomfortable about (Don't ask her that in the hot tub, save the romantic stuff for the hot tub.) and what she especially likes about your relationship.
A few suggestions for you. I wish you and your girlfriend happiness.
