Sorry for spaming, guys, I'm just full of it. Of course, rape is terrible universally, but from storytelling perspective, I can't wait for Kiruko's development coming out of this. Maru's close, he's already suspicious and NOBODY touches him in hand-to-hand, so don't worry. Robin isn't Griffith - ain't touched by fate itself - so he won't come out of this unpunished.
Now, what I think, putting this chap and previous one together - Robin SHOT Kiriko on the race track. That's where the little scar on the back of her head is from. That's why he was so terrified when he saw Kiru now - he probably had no idea she/he/they got saved by the Doc (Sakuta).
Now, why did he shot her, right? He did it from behind, no questions asked and while staying hidden, so it wasn't a misunderstanding and him "going crazy because of Haruki's body on the floor". I think - since I believe he was always messed up in the head - that he also had his way with Kiriko (the Sis) back in Asakusa. Probably also against her will or at least he "pressed her into agreeing". I think so, because no way it's his first time right now, if he prepared a bath, stole the clothes and was waiting alone with handcuffs ready - seemed planned and "rehearsed".
My guess is, he was afraid Sis would spill the beans now that Haruki was dead. He was scared of what people would do to him if they found out he was molesting or raping Sis, so he shot her in the head. Let's go one step further even - he only did this after Haruki died, because before he was blackmailing her. Like "tell anyone and I'm hurting/killing/raping your brother too" kind of deal.