I really, really, don’t understand when people say this. People can look like a mixture of their parents, they can look not very much like their parents at all, or they can look like one parent or the other. Genetics, especially with respect to race, are incredibly complicated. There are stories of two white parents having a black child, and two black parents having a white child. These are pretty rare, but it’s not that out of the question or extraordinary that an interracial relationship could produce two children who are of different races. I have a friend who is Latina and has a child who is extremely fair-skinned and is blonde-haired and blue-eyed.
It’s most definitely possible that two sisters with the same parents could appear to be of different racial backgrounds. Prim being blond-haired and blue-eyed is not at all an indicator that Katniss is white.