We are Mint

We are Mint (Mint na Bokura)  is a light-hearted romance manga about identical twins Noel and Maria Minamino (who happens to be very close to each other) and various love situations that occur around them. When Maria falls for an older guy and transfers to the school (he goes) to be closer to him, Noel - who seems to love his sister a "bit" too much - is struck by extreme jealousy. To make things worse, the school that Maria has transferred to is a boarding school, which will cause him to no longer be able to hang out with her. Since Noel won't stand a chance, he decides to enter Maria's new school as well and persuade her to come back. But there is one major problem: the only open admission left is in the girl's dormitory! Supposedly, that fact alone is enough to deter any normal, proud guy. But Noel seems to be anything but that - especially when it concerns his beloved sister. Armed with a wig (hair extension), headband, a pair of suppatsu and padded bra, Noel resolutely masquerades as a girl and begins his new life in Morinomiya Boarding School.

Wikipedia