Futago no Danjo ga Wakaranai

Takushi Hatsujou had played with a twin brother and sister when they were all kids, and he fell in love with the sister, culminating in his asking for her hand in marriage when they grew up, to which she agreed. However, they were torn apart for 10 years until Takushi is finally able to get his grandfather's permission to marry her. When they finally reunite, he is surprised to find they both look like girls. Although he knows one of them is cross-dressing, he can't tell which is a guy and which is a girl.

(Source: MU)