Found an illustration page for book eight of the Ore no Imouto novel on Baka-Tsuki which finally gave the proper names of Kuroneko's little sisters.
(Is linking like this OK here? I don't post on the forum often)
From the look of it they should be:
gokou_ruri's_middle_sister = gokou_hinata
gokou_ruri's_youngest_sister = gokou_tamaki.
Updated by jxh2154