Try kicking up your exercise a notch - Just increase the intensity, the time... Something. Keep your body guessing!
As far as your sister goes, if she's truly 5'9" and 110 pounds, she's technically underweight... I just checked it out on a BMI calculator(Granted BMI certainly isn't everything, but still!). That weight is scary sounding to me, so don't be envious of that. The goal is to be healthy, and that's what you're working towards. Don't forget that.
Your body is going to do what it's going to do... But progress will happen. You just have to be patient! I know it isn't easy, but so few things ever really are. Just hang in there!