Boy confusion

Hi, my name is Jane. I'll tell you a bit about myself, to gain an understanding off the situation. I am 17 years old and I have never been in a relationship. I've had many like 'one night flings' but i just get awkward the next day and they never amount to anything and any person who I actually like I literally fall for them and can't stop thinking about them (happened around 3 times,) and even if I know they like me and want something to happen between us for some reason I never let myself do anything about it. Anyway here's the problem there's this guy, let's name him Joe. Now Joe and I have been in the same year for the last five years. We used to be really close and in the second year and third year I found out he liked me, so automatically I started feeling myself becoming detached from him and a distance grew between us, even though i did like him. So in the fourth year we just really weren't friends and he became really 'popular' sorry I can't think of a better word. Just another bit of background, he's also known my sister for longer because his sister is best friends with my sister. My sister had always joked about how we were gonna end up together and how we would be perfect for each other. I even told her at one stage that I liked him. So fast forward to last summer, we weren't close at all, and I found out that when my sister was inter-railing with his sister they called into his family in France, and they made-out. obviously I was sorta surprised and a little bit hurt but I didn't really care because we weren't really friends. We sorta bonded again over the fact because we made loads of jokes about it and all and then we became friends. But now every time we go out to a party we spend most of the time together (not just us all the time but sometimes in a group) and I can feel all my old feelings for him coming back. I don't know what to do I feel so weird about him kissing my sister and I don't even know if he likes me but I have this irrational heart wrenching feeling when I think about him. I want to do something about it but I'm terrified and I know I won't make a move. Is it wrong since he kissed my sister? any advice? I know it's quite a juvenile problem but I can't talk to any of my friends because they will make a big deal about it.