She wasn't a sidekick, if anything, she was the instigator, the leader. Mei has always been reluctant and I can't blame her for giving in to the display of kindness and love Yuzu showed her.
No, Mei began it all when she kissed Yuzu in their bedroom not long after meeting. Yuzu is not the instigator, it was 100% Mei who started it all.
See, if Mei was the one who started it, promising Yuzu all sorts of things to seduce her, knowing full well they couldn't keep it going in the long term, I would've agreed with everybody. But in this story, Mei is more passive than anything. She still promises lots of stuff to Yuzu but it's because she's too young and too in love with her to refuse her, to say to Yuzu "keep your hopes down, relationships aren't set in stone" (in the library in vol6); and that she didn't have the heart to break Yuzu's further : her step-sister just had an existential crisis and was really feeling down. The smile she does at the end of this chapter is like a complete opposite to her crying face in the last chapter.
Except, again, Mei did start it with the kiss. Mei was also the one pushing to have sex first. Mei was the one who went on the attack the first sign of Yuzu getting with someone else. Mei was far from passive. She wasnt the one pushing for a normal wholesome relationship but she was certainly the one pushing for a physical one.
Well, the only thing Mei can be accused of is to have been seduced by Yuzu, but who can blame her for that? Okay, okay, I'm pushing it. Still, it hurts, it wasn't a kind act at all, but I don't think she is a monster, just a confused selfish teenager who still wanted to indulge in the love of her step-sister, even if just a bit.
Yuzu was seduced by Mei starting with the kiss and Mei was the one pushing for sex before Yuzu was ready for it. She is just as much of a monster as a playgirl can be. She seduces Yuzu, gave into Yuzus demands for an actual relationship and then shattered it all in almost the worst way possible. People arent saying she should be hanged but you have to see why a lot of people would hate her for doing what she did.