I wonder if it is burmese point on her. Thereby not said she is a burmese, it can be like with most others point cats - both parents are carriers of the point gene, but themselves they can have any look.
Siamese points are almost always more light in the body, if they darken up they do it first as senior cats,as many do tend to be darker with time.
While burman are often dark in the body.
Otherwise not impossible it is some sort of fever coat - especielly if she is still a kitten. Sometimes you can be really mistaken, I myself did it at least once...
The other cat is no point, just "common" dark cat, no?
My Siamese, Frosty is very similar to this. She is actually lighter overall on her body than your kitty. With dark tail, paws, ears and face (which I guess classifies her as a point).