I really like he suspense aspect, literally everyone can be your sister out to ensnare you. I'm not entirely convinced she won't opt to become a man in order to bamboozle the MC.
I like the suspense of his sister potentially showing up out of nowhere. It'd be weird if his sister reincarnated as someone older. Therefore any girl under the age of 7 is a suspect. What if reincarnates as a boy though?