If she can overwrite whatever skill she copies, that is just like the Stalker character in the game Ragnarok Online that can copy skills from another player (including healing). She can be the auxilary healer by copying the High Healing skill which can help the party out if the main healer is low on mana. And she can be the spare wizard for more DPS. But in this case, I bet the author just wants Rin's sister to go with him for leveling, so that means she could eventually copy his teleport skill. But if she can copy a unique skill, the government would want to put a ban on her traveling out of the country because she could serve a hostile country by duping and using certain national secret skills which are bound to appear and be hidden by the government.