In the series Sakurako was very sweet to Himawari. We do see her get suddenly mad and jealous once they enter middle school. My theory is that the Oomuro-ke don't appreciate her (because she's sporty and fun-loving, not a Type A student) and have soured her on life. The way her sisters don't respect her at all, even the younger one, would be a bad sign if she was a more serious character. It would also explain why Himawari still puts up with her.