Make out with this girl 2 weeks ago. Since then, been getting mixed signals and flakiness.

The good:

-Very recently asked me to be her date to a wedding in a month.
-Every time I see her, she invites me to a lot of vague future plans (i.e. house warming party, dinner with her friends, trip to Iowa for football game, etc.)
-She always initiates online chatting.
-She ALWAYS calls back or picks up.

The bad:

-She's INCREDIBLY shy, which makes it hard to flirt (difficult to tease her, she brings up serious topics alot, she gets self conscious, quiet, etc).
-She's an unbelievable flake, and any time I try and get her to 'come with' to some party, bar, event, etc. she seems to give a lukewarm response.

The catch:

She's my good friend's sister and consequently warrants a unique SPAM. He actually really wants to see her with me and I happen to really like her too. Yet, she has been single for the longest time and maybe is the cause of her unusual shyness/behavior.

So...

Any advice as to how to go about treating this more sensitive situation? Do special rules apply? What should I do until the wedding in a month? Not sure if I should be pushing any dates until then, or if I should just casually stay in contact with her.