"They follow their brother."
Translation:Eles seguem seu irmão.
If "Seu"can be used for "her" (as you indicate) then how or why do we have the word "Sua"? So far, Seu/Sua are so confusing. When and how to use. Is there a rule? Portugues needs some rules. Not just explanations such as "that too is correct" or "either way works". I understand Portugues is context based but still some form of semblence needs to be followed. Right?
English and romance languages (including Portuguese) use possessive pronouns differently.
In English you use his to indicate that the person who "possesses" is male: his brother and his sister.
The same brother and sister of a female are her brother and her sister.
In Portuguese the pronoun doesn't take the gender of the possessor but that of the person or object that is possessed: Seu pai means his or her father. Sua mãe for his or her mother.