I want to have sex with my girlfriends sister.. more to it..?

[breasal]

I have been dating my girlfriend for 5 years or more and we get on well! The only thing is we have been dating since a young age of about 15. She has slept with more people than myself which is partly why I believe I have these feelings. my girlfriend is the only person I have really slept with, where she has slept with 5 people. her sister is a couple years older than me where my girlfriend is a couple years younger. But I have and have had for quite a few months very strong sexual feelings towards her sister. I know we can't have everything in life which can suck! But I do want to experiance sex properly a couple times with another person. I don't want to live life not experiancing it! But I do not want to loose my girlfriend. I know most my replys are going to be your just wrong you shouldn't sleep with your girlfriends sister, but i'm sure years ago no guy had just one sexual partner. its in our genes to want to have sex with as many people as we can to spread our seed as they say and keep your genes going. and as i am being told countless times... we only live once this is no dress rehersal! so shouldn't we experiance the things we want? (its not like the thing i want is against the law or wrong like murder or rape)!!! is there a way I could let this person know how i feel. If they don't feel the same we forget all about it and move on. but if she does then its perfect because well we both know for sure we could never tell anyway as her family would get annoyed at her. and I couldn't tell because I would loose my girlfriend. I am not saying I want to keep a sexual relationship going, Just a couple times to experiance it! I wouldn't mind just mentioning it to her but I dont know she wouldn't tell people if she said no so I can't risk that! if i knew for sure she would say no and forget it I would mention it. So really I want a way I can sort of let he know what I want without telling her directly so if she does want to tell people then its nothing I have said straight out. any help?

p.s. i can't do anything like walk in our her or let her walk in on me because that just does not happen!

Oh and also before you mention 'would you like it if you knew your girlfriend was doing this behind your back' the answer to this question is... If I had a girlfriend who had never slept with anyone else apart from me and I had, I would give them the option to go away and sleep with someone a couple times to experiance it, if they wanted! as long as she was safe! I know I could just be saying that, but even now if my girlfriend said to me I want to sleep with someone else just once more to get it out my system, I would let her. But I know my girlfriend is not like me! at the end of the day its just sex, im not dedicating my life to someone else! and I don't want to keep doing it! just experiance it a once or maybe twice! and if I keep wanting to sleep with her sister or then other people then I know I have not forfilled my life personally yet and therefore even though I do not want to break up with my girlfriend, I need to forfill my life on a personally level before I can be truely happy on a shared level!

[burt]

break up with her, then you can sleep with whoever you want

[kyledyr]

you are definitely not ready to be in a committed relationship.
Sex is sex. If you want to experiment, go get a book and try some new stuff.
Leave your gf's sister alone.

[wal49]

If you do this, keep in mind that she will never, ever forgive you. She may pretend to. But she won't. So hopefully you're prepared to deal with that, and the fact that it would tear THEIR relationship as sisters apart. If you care about your girlfriend, you won't do it.