Normally, I don't watch two harem animes back to back like I've just done.
Nakaimo - My Sister is Among Them!
What made me start the series was the description of the show.
"In accordance with his father's will, Shougo Mikadono transfers to the school that his future wife is said to be attending. But also attending the school is Shougo's younger sister who was separated at birth, and he doesn't know what she looks like - but she attempts to get close to him without revealing who she is! Can Shougo figure out which of the girls at this school is his sister and find his destined life partner at the same time?"
But the rules of the harem genre are, as always, unforgivable. However, it does have a few nice twist and turns in it, but towards the end, you'll know who it is given the direction of episode 11 along with puzzle piecing earlier clues.