Futago no Danjo ga Wakaranai

Takushi Hatsujou played with a twin sibling and their brother or sister as children and developed feelings for the sister, eventually proposing to her after growing up, which she accepted. They were separated for a decade before Takushi secured his grandfather's approval to marry her. Upon reuniting, he discovers both appear to be girls, and though he is aware one is wearing clothes meant for the opposite gender, he cannot determine which is male and which is female.