The thing is linkat, people's body's change.
The way your daughters body works in decades to come can be completely different.
Through my childhood and into my early 30's I was very slim.
Mid 30's to mid 40's I put on some weight, but what I would consider a normal consequence of aging, like 10 to 15 pounds. I too always had a lot of toned muscle, still do.
Then after mid 40's weight started piling on. I would vascillate between figuring it was age catching up to me, and worrying/anxious there was something wrong. The only real answer I could come up with for weight gain for cortisol, because I was under a lot of stress due to husbands ill health, and other things. I ate right, exercised, and honestly, if someone told me I should "just practice portion control" I would have metaphorically smacked them in the mouth. Eating as healthy a diet as I ever did, with the "approved" diet of balance of fruit, veg, low fat, protein, plenty of water, etc etc. I would tell my husband and close friends "I am SO hungry. I have to eat something. I am honestly hungry!" I would get sinking spells, feel nauseous and anxious if I didn't eat. All my blood work was coming up fine, my doctor knew my family history, I would be honest about my food intake, and exercise level. I went on that way from mid 40's to early/mid 50's. To repeat, all my blood work during that approx 10 years was very good.
My mother died of diabetes during that time, but I honestly did not relate it to me. She never exercised, was lazy about what she ate, underestimated/lied to herself about her diet and what she could do for herself. My older sister is diabetic also, but she's been very obese since her early 30's, and inactive, so again, we seemed to be totally different animals.
What I didn't realize was that although my physicals and test results were telling one story, one that was confusing to me, my genetic background, PLUS consumption of what I thought of as healthy carbs (fruits, veg, whole grains) was working against me at a as yet unseen level.
It wasn't until maybe 3 or 4 years ago that my fasting glucose and A1C (3 month average glucose) started creeping up. I got to the point last Oct/Nov that if the numbers went any higher at all, I would be officially diabetic. However, that's all changed, because I've started eating only what people evolved to eat.
Until 10,000 years ago, carbs were something that rarely got into our bodies.
50% or more of this countries population is not diabetic or pre-diabetic simply because they "aren't controlling their portions" or other over simplified reasons. We're that way largely because we've become afraid to eat what our bodies need to burn efficiently, fat, and instead eat sugars/carbs that are in just about every food that's been packaged in some way, even when it doesn't need to be.
We are eating the worst combination, sugars and fat. Our body automatically takes the easy road and burns the carbs, and the fat gets stored. We need to be eating the fat, and allowing our bodies to burn that. Unfortunately, adding anything other than a small amount of carbs will keep us in a constant state of craving for more.
People don't want to hear that, as evidenced by the thumbs down I get, but I didn't make this up on my own. It's the way the body works.
So good thing your fit daughter doesn't gravitate towards crap. Your younger daughter needs to beware of that "I can eat anything" mode. It catches up to you.