This is the 219th most frequent Icelandic word.
sín
The Icelandic word "sín" means "his/her/its/their own" (referring to possession).
Here, 'sína' is a possessive pronoun that corresponds to 'his' in English, reflecting ownership or relation to the subject 'Hann'.
Hann elskar fjölskyldu sína.
He loves his family.
In this sentence, 'sína' is also a possessive pronoun, showing that the book belongs to the subject 'Þeir'.
Þeir fundu bókina sína á borðinu.
They found their book on the table.
Here, 'sínar' is a reflective possessive pronoun that modifies 'þarfir', emphasizing that the needs belong to 'Hún'.