@HOOfan_1 Pretty sure its because he's the older brother and its because she constantly gets herself into trouble. He doesn't just beat the shit out of her for no reason.
@HOOfan_1 Yes, while the mother disagrees, she also seems to not care enough to stop it. She simply see it as them roughhousing and thinks nothing more of it. Especially considering that
she literally left her in the chains her brother put her in one of the later chapters while they were going to her first tea party.