Contact girl

So I met this girl at a festival, we talked for a while, had some fun.

When I had to leave she said, if I wanted to contact/know more about her, I had to talk to her older sister (attends the same school as me/worked together for a while). Assuming that she said this, I didn't ask for a phonenumber or what so ever.

So my question is:
should I give it a try or is this some form of rejection, assuming she did not give me any phonenumber?
If not:
what is the best way to approach tis older sister of her, what should I ask?

Clearly, you have to pass the Big Sister Test. Are you confident that you can? Then do it. Go see the sister in person and tell her how much you like the girl, how much you are looking forward to talking again.

Then give the sister YOUR phone number. Let Contact Girl make a move.