I am a guy, and my best friend is a girl. We are close in so many ways, probably closer than most couples you'd find around, but we both insist on just being best friends for the time being. And we've had this for a year now, and I still love her as much as day 1, and things are going well. We do hug, and kiss on the cheek and so on, and nobody complains once they're used to it.
And she's your sister, of course it'll seem like she prefers him sometimes, that's just sibling dynamics. I love my siblings really. (well mostly anyway)
Let her enjoy herself and be how she wants to be with him. I'm sure you'll meet a guy like that someday, and the way it sounds, you may already know your future brother in law ;)
Her younger sis got jealous of me too at the beginning, it took her a little time to warm up to me, and now I see her as practically my own little sister. Although there are times she wants to be involved a little too much in what I do with her sister, but 9 year olds will be 9 year olds.
Everything will work out, trust me :)
Well I'm kind of relieved to hear that my best friend isn't the only one who kicks out sisters, I was getting worried. But in the end she always loves her sister, I know she does, and your sis loves you and cared for you just as much. She'll show it when the time is right. Just stick to your friends for the time being, and enjoy your youth and freedom ;)
Every cloud has a silver lining :)