Easy way out, at the start, ask for the player's name and then ask for the relationship. Such as, You are her _____ then she is your _____, then your relationship is _____ . Then we can fill out that she is your sister, you are her brother and you are siblings to fill out the space. The words we choose, are then replaced in all the text in the game. If people want her to be the "Friend" they can.