Holy crap. I don't know what to say, I'm so torn over whether to celebrate how her sisters stood up for Yotsuba, or lament how her admittedly flawed attempts at gaining individuality reached such a tragic dead end.
Because sisterly love is, of course, awesome - but being together "always" as a goal for its own sake? That makes no sense! The girls already have pretty diverse interests, and will soon have to go their separate ways to start a career (Ichika is facing this problem already), perhaps get married and have their own family... there is just no way they can stay together how they are now, and that is not a bad thing!
This chapter blew my mind. Haruba Negi, my hat's off to you for pulling this off in a harem manga.