sin / sitt / sina (for singular en-words / singular ett-words / plurals) are reflexive possessive pronouns. This means that if you have already introduced a subject, you need to use the reflexive option. If you haven't, you use the general (hennes for females, hans for males, deras for plurals). Like this:
- her book = hennes bok
- she reads her book = hon läser sin bok, and the book belongs to her
- she reads her book = hon läser hennes bok, and the book doesn't belong to her
She loves her children: Hon älskar hennes barn. She loves her (own) children: Hon älskar sina barn.
'Sina' because 'children' is plural and 'child' and 'children' are both 'barn' because they are an 'ett' word. Unless you're referring to 'the children' where it's 'barnen'.
hennes is the possessive pronoun for hon. Like, my is the possessive pronoun for I in English.
There's a special complication with the words for her, his, and their though – if 'she', 'he', or 'they' is the subject of the sentence, we use the possessive pronoun sin instead. So we get Hon älskar hennes barn 'She loves her child' (not her own, some other woman's child) but Hon älskar sitt barn 'She loves her child' (her own one).