Difficult situation, advice needed

Hi, I'm new to this forum but I have a quite tricky problem that I really need help with cos I dont know what to do.
Right, here goes. I found out today that a girl I know really likes me. Normally Id be pretty happy about this, but this time Im really stuck and I dont really know what to think. Thing is shes 15 and Im 20. Bad start. Shes also the sister of one of my ex-gfs whom I've stayed friends with and who now means a lot to me as a friend. Not only that, our familys are good friends as well. My mum went to college with her dad. Was down at her house this wkend (as I do sometimes, Im friends with them all) and she was being quite friendly.

Examples:
Always sitting close to me preferably touching me with something (like a leg or just sitting so we're touching).

Making excuses for other ppl to leave the room so that we'd be alone.

Touching me in a flirty kinda way (you know, touches to the shoulder, hair, leg etc... to emphasise a point)

Getting angry when people would take her seat (next to me).

Now, I wasnt doing anything to actively lead her on but I wasnt discouraging her either. To me, it just seemed like she was being playful/flirting like teenagers do (opinions on this theory please) but my ex noticed and pointed this out to me. I really dont want to hurt her. The whole family means a lot to me and I do care about her, but this cant happen right now for obvious reasons. Theres way too much at stake. I need some advice on what I can do to let her know this. To let her know that its not bcos I dont like her (cos I do). Its not bcos I think shes unattractive (cos I dont). I dont want to wreck her self-confidence if you see what I mean. Anyway, any advice would be very much appreciated. Thanks.

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