<human>: Can you provide a brief rundown of the text in within 3 sentences?

I'm 21 and I have a sister who is 6 years younger than me at 15. She and I have always been close and like to hang out together. I'm usually off at college but now I'm home for summer and we're hanging out again. She has some friends, though, that she likes to bring over and they are all nice girls and I know all of them very well as they have been her friends for years and I've known them since they were a lot younger, but now they are getting older and are kind of in the territory now where they are trying to hang out/flirting/trying to be actual friends with me and I'm not sure what to do without either being a jerk or being a creep. 

For example, whenever I'm online I'll almost always get a message from one one or two of them asking what I'm up to and I'll try to be polite and talk with them and stuff for a bit before lying and using an excuse such as "have to go homework". Usually they'll drop a semi-flirty line here or there and I'll just ignore it. I'm not quite sure how to handle any of this. On the one hand, I don't want to be anything *near* romantically involved with any of them **at all**(emphasis to get the point across). On the other hand, the few that try to talk with/hang out with me are actually really cool and nice and I wouldn't mind being friends(?) with them if they wanted to, but I'm not sure that's good for them or me.
<bot>: what do I do about my sister's friends trying to hang out with me without being a 21 year old creep but without being a jerk to my sister or her friends