The other half of her spawn are also girls.
What Hejira's saying is the woman has three children. All three are girls. This means that half of them are girls, while the other half are also girls.
This is really a "pseudo–riddle", similar to the one that goes, "I have two coins that add up to thirty cents. One of them is not a nickel. What two coins do I have?"
They're not really riddles imo; they're fairly stupid "trick questions".