If your looking for an idea you could for have the brother over the course of several days or weeks take things fr m his sister at a rate slow enough that no thinks anything of it. For instance he could be tired of her talking one day and decides to take parts of her vocal chords until she can no longer speak and everyone can just think its a cold. The same could go with muscles. He could be tired of losing to her and decides to start taking parts of her muscles away and everyone could just say she needs to work out. Maybe find some way to messw ith her brain or other organs. Maybe even replace organs with different things.