Teenage Girl With Boy Problems

Hey guys.
So, today I'm gonna do something I've never done in a blog. I'm going to ask for advice.. About boys. I'll explain my problem in a second, but first, some background.

I am a girl who doesn't date. Yet. That's not to be confused with I've never been ask out, because I have, I just want to focus on school and stuff and I think dating can wait. But that doesn't mean I don't have crushes or flirt. Also, my reputation isn't a good as I wish it was, due to a few things.

1. Most of my friends are boys.
2. I'm a trainer for football and baseball. (Not sure where we got such a bad name!)
3. I've had a bad expierience with sexting. (What girl hasn't?)

Anyway, that's me. Here's the boy.

He's in my sisters grade, so he is two years older than me. Making him 18. He's also friends with my sister. He also has a girlfriend. Trust me, normally I do not think about other people's boyfriends like that. And it's not like I'm planning on stealing him. I just don't know what's going on. Let me explain.

1. The other day at practice he ask me to get some ice for his leg, and on the way to the field house he treated me like we were old friends.

2. The day after that he was in the trainers room icing his leg, and he jokingly kicked me and chatted with me.

3. Later that day my sister and I went to the softball facility so she could pitch. (I was gonna catch her.) He and and my sisters boyfriend were there, along with some guy I don't know. He and Chase, my sisters bf, we joking around with me and generally making fun of me. (Not in a mean way, like in the way that you laugh at yourself.) He invited Alex to a party he's having, am said I could come, too. "Just bring one of your boyfriends." That's what he said. That's what he said! What's that supposed to mean?

Thanks guys. And I'd just really love you to shed some light on the situation. By the way, I'm not going to the party. I'm having a friend over.