If she feels bad about killing people, then she can just kill the stepmom and not kill the stepsister. Then she would, in theory, only feel half as bad as she would if she killed both. The stepsister wouldn't be able to hurt her without the stepmom's backing. Plus, it is a kill or be killed situation and the staff in the mansion are also in danger. So, my moral compass tells me she would still have the high ground.
If she feels bad about killing people, then she can just kill the stepmom and not kill the stepsister. Then she would, in theory, only feel half as bad as she would if she killed both. The stepsister wouldn't be able to hurt her without the stepmom's backing. Plus, it is a kill or be killed situation and the staff in the mansion are also in danger. So, my moral compass tells me she would still have the high ground.