Siya is an indirect pronoun meaning him/her, but niya is a possessive pronoun meaning his/hers. "Malakas siya." = "She is strong." "Mahal ang kotse nya." = "His car is expensive."

Exactly so. Personally, I find direct comparisons especially helpful, so: Magaling siya = He/she is talented "Ang galing niya" = (literally) "The talent of him/her", which is a common way of saying "He's talented" in Tagalog.