J.W.
The best way I had of learning more about my younger daughter after she quit being a chatterbox was often while she was sitting in the front seat of the car with me driving her places. I would chat a bit about nothing important, and she would start chatting... oh how nice that was! TRYING to get them to talk, in my experience, does NOT NOT NOT work. We had some conversations around the dinner table, as well (we always had sit down family dinners).
The relationship between the two sisters was very important. It got bad temporarily, but now it is back to the way it was, and they talk a lot to each other (and me... I guess there really is no keeping secrets from between the 3 of us - but they are adults).
Trust the relationship you have built before, and the relationships with her and the siblings. My biggest confidants at age 15 were actually my 12 year-old brother and my best friend... NOT my Mom, whom I loved, adored, and needed ( still do :-) ).