OK, I flirted with this girl for a week and we got really touchy and giggly but it didn't go past that. On the final day about an hour before she had to leave I wispered in her ear is she wanted to make out later when we got some alone time (stupid I know but I was desperate... I'm 16) she said that she didn't know. Eventually she left and we didn't do it... then I learn that she told her older sister what I said now I'm so embarressed and feel like an idiot, I'm seeing them again in 3 weeks. How can I make it less awkward when I see them but keep flirting with the girl?

Why would you feel like an idiot that she told her sis? Sister's share stuff like that. I think if you feel awkward it's because you're embarrassed. CHOOSE not to be embarrassed and you won't feel awkward about what you said.

Always know that sister and bff's share this kind of thing. That's what girls do... lol

If you like her just continue to act like you do. Be respectful NOT pushy...

How old is the girl?

You are probably the only one bothered by this. It's not that serious.
Are you interested in the girl or only making out with her?
How old is she.

If you are embarrassed by making out then you are either too young or too immature to be making out. Stop worrying about it. Almost everyone makes out sometime in their life.
If you are embarrassed that you made out with her in particular, and don't want anyone to know, then you shouldn't be doing it at all with her.

You are probably the only one bothered by this. It's not that serious.
Are you interested in the girl or only making out with her?
How old is she.

She's a year younger so 15.
I'm interested in her, not just making out.. but at the time I didn't think I would see her again and I wanted to do something special, but now I'm seeing her again in a little under 3 weeks, and now I think its going to be all awkward and we won't be able to get back to, fun, giggly flirting like it used to be

So you were fine with making out thinking it was a "one night stand" so to speak but now that you'll be seeing her again you're nervous?
I suggest you forget the making out part and get to know each other. What a concept huh?

Be normal. Act as nothing happened. Apologize to her personally then just say let's get to know each other better and have a good time. If you want you should kiss her on the cheek. Remember girls flirt but may not actually like you. It's crazy I know. Girls will flirt to just feel good and wanted and not really anything to do with you. But if that's not the case act normal laugh with her still flirt as before. The more you show that it's awkward the more she will feel distant from you. Kiss her on the cheek let her know your open. You probably scared her with the makeout thing. You don't do it like that. You kiss her on the cheek wait for a response then you get into it. It's natural.
Hope I wasn't too harsh :-) good luck!