Chronological order of Kiss x Sis anime and OVA The series producers decided to move the more &quot;spicy&quot; manga chapters into the OVAs, and because of this, they are chronologically spread between the anime episodes.
What should be a chronologically correct viewing order of the OVAs + anime episodes?
After rereading and rewatching it, I conclude that the animators just took a random chapter of the manga and put it on anime anywhere they like, there is no chronological sequence of the anime, you can watch it from the anime randomly and will not lose the plot, unless for the last three episodes.
However, there is some important event that can be sorted chronologically based on manga chapter:

Manga chapter one and anime episode one when they're still kids and show how Ako and Riko save Keita from a dog.
Manga chapter five and the beginning of anime episode four (4 years ago) when Ako and Riko start to falling in love with Keita
End of anime episode nine when Ako and Riko made Keita a good luck charm
Anime episode 10 when Keita take an exam and after the exam
OVA episode four when Keita, Ako, and Riko go on a trip after the exam
Anime episode 11 when he receives an acceptance letter from the school from Mikuni (unlike in anime, this is Keita's first meeting with her on manga)
Anime episode 12 when Keita get to the same school with Ako and Riko

And from here, it only happens in manga and hasn't been told in anime, BEWARE SPOILER

 Manga chapter 42 when all the characters go on a trip and manga chapter 67 when Keita starts dating Kiryuu Sensei

Some of the OVA episodes happen before anime, and some chapters in the manga didn't happen at all in anime or OVA.
So, with that being said, this anime doesn't really care about the plot and focus only on the ecchi-almost-hentai theme. You can watch it however you like and just enjoy the show.