I liked Yuu and Mari since they first appeared but i guess its cuz from the start i like weird relationships like that and i know a bit about both S&M mentalities, i can see how people dont get their relationship and find it very weird especially at the start where Yuu was just shown as a super S, but the author did a great job in making both very likable and making Yuu the vulnerable and insecure one while Mari just shows pleasure, anticipation and acceptance to whats done to her lets people not be so weirded out and understand them better.
I looked to see if there were any prior hints at Honoka and Kaoru being sisters but nope, aside from both being tall and pretty there was nothing, it was definitely not planned lol.