Translation:Witches use magic to make breakfast for their cats.
Danish has a different set of pronouns when something is being owned or belongs to by whomever is the subject of the sentence. These behave like the possessive adjectives and pronouns for the first and second person (min/mit/mine and din/dit/dine), in that they decline corresponding to the item being owned (sin/sit/sine).
To better understand this concept in English, one can imagine adding the word own after the possessive adjective:
- Manden læser sin avis means the man reads his (own) newspaper while if the man was reading someone else's newspaper, it would be manden læser hans avis.
This extra dimension only comes into play for the third person singular, and can be helpful in distinguishing to whom exactly an item belongs. However, it takes a bit of getting used to :) Try to determine if an item belongs to the person performing the action in the sentence (the grammatical subject) or someone else. This other person could also be mentioned in the sentence, but does not carry out the action. As mentioned above, you can perform a test in your head by inserting own after the possessive adjective (in English): If it sounds weird, you should not be using sin/sit/sine.
To answer your question: it's not sine because the subject, hekse, is third person plural.
It would be sine if the subject was third person singular, such as en heks:
- En heks bruger magi til at lave morgenmad til sine katte. A witch uses magic to make breakfast for her cats.
The Danish reflexive personal pronouns are tricky - also for Danes.
Swedes use sin/sitt/sina for third person plural subjects, but that's a different course...