Will a person be rewarded for advising his sister who severely beats and verbally abuses her two daughters, while he himself treats his own children with gentleness and kindness?

It is permissible for a father or mother to hit their children for discipline, based on the hadith about hitting children for abandoning prayer at the age of ten. This hitting must be restricted to ensuring the safety of the one being hit; hitting that leads to harm or damage is not permissible. Care must be taken not to hit the face or vital areas, and one should not discipline while angry.

What your sister does—brutal hitting and cursing her young children—is Haram (forbidden) شرعًا (Islamically), and it causes them psychological and emotional harm. As for your mercy and compassion towards your children, you will be rewarded for it. A balance must be struck between firmness and leniency, with the guiding principle in interaction being mercy.

Children are a trust, and parents are responsible for them before Allah. The Prophet (peace be upon him) warned against neglecting this trust.