I hate to say it... *strain* *fights inner demons*

*gasp* ....This could become one hell of a threesome.

>>>>>>>>>>>>>And back to sincerity. (but seriously, consider the threesome)
You have three extreme loyalties here. To your sister, to your best friend, and to your self. This is a very complicated matter of hearts.
However, a couple things come to mind. You said your friend "liked" your sister, that you are "in love" with your friend, and your sister is only sexually attracted to your friend. "likes" can be swayed, but "love" runs too deep to simply shrug off. If you truly love her, letting her date your sister will most likely severely fuck up your relationships with both of them.

>>>>>>>It sounds like your sister isn't the problem. The problem is you and your friend. If the relationship didn't work out, would you still manage to be friends? Maybe you could ask a hypothetical question to her.
>>>>>>>>>>Has your friend shown any deep interest in you? Have you ever gotten "close"?

>>>>>>If I were you, I'd cut away all the bs. I'd get your sister and your friend in the same room together, (get some weed too), and tell them flat out, your feelings about the situation. Theres no other way around it. You can't expect them to have the same degree of honesty, but if you go out on the limb for two people who care about you and who you care about, they'll most likely open up as well.
>>>>>Afterwards, whatever scenario becomes a reality, you can always smoke a few bowls together and ponder the lives you'll live together, no matter who's shagging who.
>>-Inferius