That assumes Gruelle and Gruhilde age normally. Also assumes they aren't clones.
Both are possible, despite Gruelle saying she's 13 years old. She had meet Marika's father before. While that shouldn't take much considering he just died and Marika isn't much older than Gruelle. Jenny has meet Gruelle before, so we know the Princess goes to important social fuctions.
Yet there is still something missing. Something about the Princesses and this colony ship. Something Gruelle would like to do alone, but Marika won't let her.
(Also it is Grunhilde de geso~)
There are some fictions that subspace differently than others. Space Battleship Yamato III had subspace submarines that "submerged" into subspace to say hidden from real space sensors. They could even fire torpeodes from subspace to real space and attack (though they needed a periscope to poke into real space to aim). Mind you that Space Battleship Yamato is usually refences as World War II in Space, in terms of how things work.