I think she's purposely acting like she's weak cuz she doesn't want her family thinking she has the power that she currently has, and although she used it in front of her sister, she seems a little dense and doesn't seem to have realized it so it's for the sake of getting away from the family. If they know she has those powers, thy will likely keep her in the family to use her to their disposal (is what the FL fears). So that's why she needs a knight to "protect" her.