transfer her memories/soul into any dead body, as long as they don't have one of those Spiritual Spirits like MC (aka., only 71 other people). And the maid isn't MC's real Sister, but is a dead body turned to Zombie which she is possessing and is even able to use the Fire Magic that dead girl could originally use. Guess that means, even MC's mother could become her target for possession next, if she is able to kill her off somehow.
No wonder, i heard before that MC ends up become something akin to a Demon King, and goes on a spree to kill/destroy all women around the world (except a girl killed by the Sister, whose body MC supposedly keeps frozen), so that his Sister couldn't use them.