Best friends sister!

Ok so I've got this huge dilema! I've know my best friend for a few years now, and a few nights ago I got a feeling. I really like her younger sister! I've met her before a few times through the years and I've noticed she was really cute, but the other night we were all partying and me and her sister whom we'll just call K for now were talking most of the night, and I can tell by the way we talk she is somewhat interested and the big roadblock in my road is the friend. I understand it's her little sister and all and she's not to happy that me and k are talking on a daily basis. Now I don't want to rush into anything with anyone let alone k, I really like who she is and how she acts and I would love to get to know her better and maybe one day be with her if that's where it goes. And I know it's a problem for her that I'm her sister best friend but I want that to not be a problem, I know I could get it through my friends head that it's not really her place to dictate who I date or who k dates because we are our own people with our own minds, but they even have an agreement not to hang with each others friends without the other one being there. So I'm not sure how to ever get her alone so I can try to maybe build the friendship me and k have to a level where dating would be an option. I mean we've talked about it before, that it could get to that level and she doesn't seem to be against it at all. So I'm really at a roadblock with how to pursue k, I want to get to know her better before I try to be with her, but I can't even be alone with her and my friend will get in the way if she's around. Any ideas would be appreciated

are you sure that your best friend isn't harboring any feelings for you? i think it's really strange that two sisters would make an agreement to not hang out with the other's friends alone. maybe the younger sister knows that her older sister has feelings for you and told you about their "agreement" to give you a bit of a hint. personally, the way i see it is, if one of my best guy friends had feelings for my younger sister, and i knew they were legit (not trying to get in her pants), i would be more than happy to give them alone time to get to know each other. since he was my friend in the first place i'd trust him, i would trust him more than some random guy i didn't know dating my sister. so i'm thinking that there is something else going on here.

Actually me and my friend have talked about these things before because I useto hav feelings for her and it really wasn't that long ago we talked about it, an their agreement was made years ago. So that isn't it