What do you want? Your girlfriend or her sister? Because you cant have both. If you want your girlfriend then you have to avoid the sister. Put as much distance between you as possible. It's not appropriate. If you want the sister then you have to be honest with your girlfriend.

Can I start with saying "you sly, underhand, charismatic devil "

Lol

In all seriousness though, I see the dilemma you face. You have your beautiful girlfriend, but you then have her 20 year old, hot sister. And the temptation "be a burden to a mans loins".

Has her sister reciprocated any attraction for you? Is she giving you those tiger eyes, light touches and brushes as you stand near her?

If yes, then I would argue that half of your attraction is being instigated by her, and I would have an immediate word with your girlfriend. Just tell her what you think her sister is upto. It might cause tensions, but at least its out in the open and you most likely won't feel horny any more (for her sister).

If this is based purely on your own desire and her sister isn't instigating anything, then I would consider perhaps ending things with your GF or maybe changing the arrangements of your relationship to something more "open".