I really want to believe that this manga won't just deteriorate like the last two manga I read about gorgeous sisters (one of them involving a twin), but the fact that a "little sister" already showed up is a bad sign.
Let me guess: it's going to miss Masaki all of a sudden, probably in some obnoxious >90% tsuntsun manner, and so begin the harem for the sake of "variety". After all, this manga is about two gorgeous girls, and one of them even has slightly softer eyes than the other, with a submissive demeanor to match--can't have them be the focus alone, right?
Masaki even started the tsukkomi routine not long after Fuuka showed up--and it even spread to the same Yuzuki that he was so smitten by. A bad sign, but at least he's not backing away from the two.
I hope for the best, but I'm going to temper my expectations.