I hope the MC gets with his reincarnated sister. They are actually perfect together. The MC in his past life was a weak loser coward. So he needs someone strong and crazy like his sister to protect him and give him the sexy times. I mean, he had free, fully working hands and was able bodied, but watched his sister kill, what I can only assume is more than 1 person in front of him, yet he did nothing. The door was probably never locked in the first place. But he just stayed in his room, because he was scared, not of his sister, but the outside world. He doesn't get any of my pity or sympathy.
Yup, they are perfect for each other. It would be a perfect ending if they get together at the end.
But seriously though, without the yandere sister reincarnation premise, this would just be another third rate isekai.