They lose their parents and sister is terrified of losing him so she becomes overbearing and controlling, pushing him away.There seems to be a lot of missing (though implied) information.
-Consider that both the leads seem to only have 1 sister as a family contact.
-Last chapter Sasaki said "In the past, my family used to (emphasis my own) call me 'Toichi'."
-This chapter Tabata says his sister became strict with him since they started living together (with him being a 2nd year she's around 7 years older here).
Neither of them know it about the other, but my theory is that there are no living parents involved here.
Hopefully we will get to see them build a healthy loving relationship. Don't get to see that too often in manga. Though it will probably take a decade at this rate.