15 year olds are notorious for this kind of behavior - she may love it, but find it hard to express thanks.
Then, too, her Mom picked out the colors and so on, so the quilt might be what Mom THINKS she likes, not what SHE likes. Mom's are notorious for that kind of behavior! (LOL)
Before making her brother a quilt, ask him if he wants you to and what he'd like. You have a much better chance of making a quilt he likes since you know him.