Mei was sexually molested when she was younger.
One reddit theory postulates that Mei was actually molested at a young age prior to becoming Yuzu's stepsister. This would make sense in some ways. For one, it explains how Mei is very knowledgeable on sex; two times in the manga and anime, Mei propositions herself before Yuzu the second one indicating that she had done something of this sort before. It even gives more insight into Mei's more passive side. If she were to get harassed or someone forces themselves onto her, rather than resisting, she complies to it. It also does shed some light into how Mei would act coldly around others, or be at a distance.
- Her getting forcibly kissed by Amemiya reinforces this theory. Given how he did it without any hesitation, it likely wasn't the first time Amemiya did something like that. And who's to say that he hasn't done worse?
The title "Citrus" traces back through fanfic to "Cream Lemon".
In fanfic terminology, a "lemon" is a shameless smut fic. This term comes from the hentai OVA series Cream Lemon which had a fairly large proportion of yuri relationships, especially among its recurring plotlines. As a branch off of "lemon", the term "citrus" began to be used for less smutty fics of a still risque nature (roughly analogous to the difference between "hentai" and "ecchi"). I realize it's a longshot, but (aside from being what Wild Mass Guesses are for) there is a thread that connects these terms and even though I don't know much about the Japanese fanfic community, I figure there's a pretty decent chance these terms are used there, since they originated from a Japanese medium.
Resolving Chapter 36
As the thirty-seventh chapter is due to be published in April of 2018, many people have conflicting theories in regards to how the previous chapter would be resolved, or if it leads to something completely going against expectation. In Chapter 36, Mei left home in order to physically admitting to Yuzu that she was in another arranged marriage. Given that the manga is criticized for its cliché tropes, there are two solutions: either Mei calls off the arranged marriage yet again - even if it damages her chances of inheriting the academy - or Yuzu interrupting the marriage at the last minute to make her younger sister reconsider. Some had even suggested that Mei could potentially decide to runaway with Yuzu, which would again destroy her chances of inheriting the school.Alternatively, the writer of the yuri comic could also go against the readers' expectation, and end the manga on a bittersweet (or outright depressing) note.
Harumi will start dating Yuzu.
Given the forlorn expression she gave to Matsuri when Matsuri asked her if she was in love with Yuzu, Harumi could take advantage of Mei's absence as a way of getting closer to Yuzu. Could also be seen as a potential Take That!
towards those who ship Harumi and Yuzu.
There will be a quarrel between Mei's father and grandfather.
- Given how Mei's father learned that his stepdaughter was in love with his daughter, he would most likely engage in a fight with the grandfather who may despise the fact that his granddaughter is in love with someone who represents everything that the school is against. Given how his son up and left his school, Mei's grandfather may double down and try to prohibit Yuzu from confessing to Mei.