How tall is your sister and approx. what does she weigh and what kind of riding does she do? I am 5'4" and around 125-130lbs. The KX100 was my second bike and I really enjoyed it. I have never ridden a 125 or an 85 so I can't give you a comparison. The 100 is not going to be as powerful as a 125 obviously, but the power to weight ration is great being that the 100's dry weight is 150lbs. The bike chassi is just an 85 with bigger wheels it would feel similar to a regular 85. It sounds like a great step up. I rode the 100 for 2 years before I felt like it was time to step up to a full size bike. You can always have a 125 lowered too. The full size bikes are going to handle has good in the turns as a bike with a shorter wheelbase, but it will have more stability and she will be able to ride it faster with the better more sturdy suspension. A 125 will still handle great.