You can magical get fat. You can change the way you matabilize food, you might retain a little extra water, other than that you aren't gaining more fat than the calories you eat.
This country is gotten so fa,t it doesn't know what fat is. I had a conversation with someone complaining becasue his BMI said he was fay. I said 'you are fat'. He looked at me is disbelief.
I'm like, yes you are fat, not obese, but id you stomach is not smaller than you chest, you are fat.
Rule of thumb, may not apply to people on the extreme of the bell curve.
Right not people seem to think you aren't fat until you get some roll which is typically obese.
Anyways, you sister in law is using her medications an excuse to eat too much. Probably they zapped some energy form her and she doesn't move as much, thus needing fewer calories. That was speculation.