Much as I wish MahiMahi were actual twins (and much as I think Kishi-sensei wants the readers to basically view them that way while avoiding the letter of whatever editorial or other pressures it might have caused) the character summary page from vol. 6 calls them "擬似双子," which means "pseudo-twins."
Since there seems to be some question on it, that page also lists the class years for all the characters:
1st Years:
Mahiro
Mahiru
Yuu
Mari
Nao
Mayu (Kaoru's kohai toy)
Mio (Ichinose)
Mariri
Midori (new character later in vol. 5)
2nd Years:
Ai
Chie
Michiru
Airi
Ayano
Miyoshi
Honoka
Alicia
Onoda (she isn't given a first name)
Yumimi
Mayuyu
3rd Years:
Kaoru
Shizuka
Nononon
last edited at Apr 18, 2016 11:26PM