2013/9/19 'CuSO4' Yung <cuso4@cuso4.com>:
>> 08) Galilei Donna (10/11)
>
> Another seiyuu guessing time !
>...........
> Of the 2 remaining cast, one of them is either Ookubo Rumi or
> Tatsutatsu Ayana (their voices are very similar).  I cannot figure out
> who the other one is.

They just announced the cast earlier today.  The 3 sisters are :
Ookubo Rumi
Shindou Kei
Hidaka Rina.

Ref :
http://www.galileidonna.tv/staff/

-- 
'CuSO4' Yung