Il déteste son frère. He hates his brother. but how would you change this to be the french for he hates HER brother

Rona, The sentence would be Elle déteste son frère. The elle would indicate her. SON won't change.

Are you sure Son doesn't change to Sa as you are describing gender possessive, and the brother being the possessive to the Female makes Sa?

"son" agree with the gender of the noun "frère". So it's right to say "Elle déteste son frère."

He hates her brother. The subject is he, il. the object is 'her brother'. brother is masculine, so would be 'son frère'. So if I understand rightly, He hates her brother would be Il déteste son frère.

That's a good question.