This chapter is either: Risako is in love with Kaoru, but — like Kaoru couldn't with Reichi — can't express her feelings. So in order to finally break Kaoru's crush on Reichi and, ultimately, lead Kaoru into her arms, she breaks her heart. Which is what I assume she's still trying to do by having sex with Reichi behind her back. Kaoru finds out, Risako (somehow???) successfully denies all relations, profit???
or
Risako thinks of Kaoru as her plaything. She enjoys watching her emote, something she's admittedly poor at, so she toys with her.
I don't mind mangas where "everyone is lesbian." That's something that does happen, and which I had happen to me and my friend group. Suddenly, all the gay (or bi). But this is... bordering on obscene. Risako, up until this point, has displayed what I can only describe as cold amusement in regards to Kaoru. Unless I've missed some context clues, I haven't seen anything to indicate she's in love with Kaoru until some very subtle hints in this chapter. If the setup was there from the start, I could find this a little more believable, but even still this is very obviously going to be used as nothing more than a plot device to push Kaoru and Uta closer together. Which, I'm all for, I've been Team Uta from the beginning, but this feels like a cheap way to make an already overly long love drama even longer.