could just be she was the runt of the litter, therefore smaller than her brother to begin with, then also being female may stay smaller than her brother anyway. if shes eating no problem and isnt looking poorly, just give her time and she should fill out with a growth spurt.
my cat had a litter few years ago, i kept 2 and the other 2 went to a couple of neighbours so i see them often. for litter mates they are all very different. my 2, the only male and only short haird female, are both of similar build, what i would call average size, then of the other two girls, both long haird, one is still slinky slim, and the other is a pudding!!
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