I liked teko's comment to be patient and enjoy the game. If you still want a clue but want to do something that seems lower-threat than asking her to a movie, you could give her a small gift, say a casual neck chain in silver (nothing that indicates what used to be called "going steady," though. In style, it has to be more offhand-looking than that.) With Christmas coming around, it would be a natural enough thing to do since she is your friend's sister, and cute sterling silver chains are available at all the stores for not that much. If she makes it a point to wear it, then you have a clue. THEN you can ask her to a movie.
Lee? How old are you man? Look pal, teko nailed it.....enjoy it. And chubby? Heck man, chubby is the new thin....(not really, but you can get rid of fat-not ugly so you got that going, right?)
Take it easy, dont press the issue, but try to hang out more often and see where it goes. The worst thing that can happen is that she just wants to be friends at this point right? Let the friendship develop and see what happens. Like cheshchesh said, see whats happening and invite her to a movie or something else kind of fun....
I would sit back and enjoy the attention for awhile. You and she are enjoying it so be patient and enjoy the game. Plenty of time to find out where it leads. Why spoil the fun.
She's not giving you UNfriendly signals, anyway. It doesn't sound like she's "just flirting" if by that you mean is she being insincere. (If she were doing that, her behavior would be more heightened, and you are describing someone who is relaxedly enjoying your company, not shrieking and giggling and stuff.) So if you did ask her in the way cheshchesh said, which is clear but low-key and not a huge deal, and she were to not want to, she would be able to say no gracefully and the two of you could still say hello and things without embarrassment later.
well you like her and she def seems to like being around you so why dont you just ask her if she would like to do something with just you like a movie the only way too know is to ask,good luck