"The girl likes her cups."
Translation:Jenta liker koppene sine.
din/di/ditt/dine means 'your' or 'yours' however sin/si/sitt/sine implies to the subject of the sentence - Gutten spiser maten sin. The boy is eating his food. - De liker bøkene sine. They like their books.
so the meaning depends on who you're talking about
I recommend reading the "Tips" section on the Duolingo website, it usually explains things like this
No: 'sine' here implies that the subject, the girl, owns the cups, and not some other girl/woman. Were 'hennes' to be used it would imply that the subject didn't own the cups, but some other girl/woman.
Jenta liker koppene sine. = The girl likes her own cups.
Jenta liker koppene hennes. = The girl likes the cups of someone else.