That uniform looks really nice on Kobato, I don't doubt that both Kirino and Kuroneko would love to have her as their little sister.
Great, now I'm imagining a yuri threesome doujin of Kirino and Kuroneko having their way with Kobato while trying to get her to tell them who does it best.
I think (s)he's referring to the fact that both Kobato and Kuroneko have an obsession with a gothic lolita character. Any degree of likeness can be used to pair up 2 characters.