In love with straight best friend who's interested in my sister! Help!

starts as a familiar situation: i'm gay, 22 yo and in love with my 21 yo male best friend (who knows i'm gay and likes him). a few things have happened between my friend and i which gave me hope that he could be bi and possibly start a relationship with me. but i think that realistically he's just straight and won't get involved with a guy. i know that i should just concentrate on our friendship and get over my deeper feelings for him and focus on finding someone gay or bi who can love me the way i want my best friend to.

my friend has had a few girlfriends before and i have tried to be supportive of them but my jealousy made me try to avoid talking about his girlfriends. i thought "ignorance is bliss" and if i don't have to see or talk to those girls then i don't need or want to know what goes on between him and his girlfriends (thinking that their stories would make me crazy with envy).

now i'm faced with a slightly different problem. my friend is now single. and so is my 19 yo sister, even though she is currently dating a guy. i live at home with my parents and sister. when my friend and i get together, it is mostly at my place. sometimes, of course, my sister is home too and my friend and her talk to each other. she even joins my friend and i sometimes with whatever we may be doing at my place (video games, drinking etc). i'm worried (heck, scared even!) that he and she will get together as a couple. my sister said a few years ago that if he was single she'd go for him (he had a gf at the time). i don't think my friend has mentioned any interest in my sister but i'm pretty confident he is (she's attractive kinda similar to one of his ex-gfs).

i'm upset because i don't have many close friends (2 to be exact) and my sister is a much more sociable person than i am and has more friends. i think she should stick to her own friends and leave me mine (i guess that since i don't have a lot, i defend what i have more passionately). also, i can deal with my friend having a girlfriend, as long as i don't have to see them together (when my friend and i hang out it is almost always just the two of us and i want to keep it that way). but if my friend hooks up with my sister then i'll be living in the same house with his girlfriend! i'll have to see his gf daily and even try to still like her (i feel close to my sister even though my jealousy now also makes me resent her). i also fear that if he's with her he'll have less time to spend with me. it would also kill me knowing that i was the one who brought the two of them together. such bitter irony that'd be.

i'm afraid of telling them that i don't want them to hook up because i don't want to lose or damage my relationships with them. but i sometimes feel like i'd have to give my friend an ultimatum: he either chooses me or my sister. i know that as a friend i should care about my friend's (and sister's) happiness, and if being together makes them happy why should i try to stop that? i guess my plan is to try and keep them apart when i can (selfish, i know) but accept that i can't help it if they do enjoy each others company and become closer as friends or a couple. i guess all i can strive for is prevention because a cure would probably destroy our relationships.

what i'd appreciate are different points of view, suggestions, advice, stories from guys who've gone through similar ordeals etc.

thanks a lot for reading, and for whatever you have to offer