“Siya”, “niya”, and “kaniya” are third-person pronouns.
“Siya” is the equivalent of “he” or “she”.
Examples:
Sino siya? = Who is she?
Siya ay si Anna. = She is Anna.
“Niya” and “kaniya” are possessive pronouns, equivalent to “his” or “her/hers”. “Kaniya” becomes “niya” when it is placed immediately after the thing that is owned.
Examples:
Si Mary ay naglinis ng KANIYAng BAHAY. = Mary cleaned her house.
Si Mary ay naglinis ng BAHAY NIYA. = Mary cleaned to her house.
(Sa) KANIYA ang LIBROng ito. = Ang LIBROng ito ay (sa) KANIYA. = This book is his/hers.
LIBRO NIYA ito. = Ito ay LIBRO NIYA. = This book is his/hers.