Himawari and Sakurako can't stand five minutes without tearing each others' throats off.
And yet those two spend most of their time together, be it at school or out of it, Himawari keep helping Sakurako with her homework, whenever she cook some snacks she set some aside for Sakurako with some special wrapping and on occasion, even Sakurako will be nice or helpfull to Himawari (though she's usually on the receiving hand). Also, they filled a marriage registration form when they were kids and even though they apparently forgot about it, they still remember the reasons behind it.
Those two are extreme tsundere toward each other, but on the rare occasion where they actually fight for real, they usually feels bad about it right away.
If they really couldn't stand each other, they would avoid meeting out of class and council duties.