Son, sa, ses follow the same rule as mon, ma, mes: they agree with the thing owned. So they can mean his or her — the word itself doesn't tell you which:

Thing owned	Form	Example
masculine	son	son père — his father / her father
feminine	sa	sa voiture — his car / her car
plural	ses	ses enfants — his children / her children

Marcos's father loves sa voiture; Marcos's sister loves sa voiture too. Context tells you who the owner is.

And as with mon amie, a feminine noun starting with a vowel takes son: son amie, son écharpe (his or her scarf).