Thanks for the new chapter!
I am ashamed of myself for only realizing until now that the names of three sisters had numbers in them:
Kazuki - 'kazu' is another way to pronounce the kanji for 'ichi' or the number 1 (the same is used for Kazumi, Kazuya, etc)
Niko - 'ni' is simply the number 2 (can also be read as 'futa' as used in futari, futatsu, futanari, etc)
Miwa - 'mi' like kazu is another way to read the number 3 (as used in mittsu)
Just like in Gotoubun no Hanayome (Ichika, Nino, Miku, Yotsuba, Itsuki), these numbers in their names are used to indicate the order of their birth.