According to my norwegian classes, si-sin-sitt are 3rd person posesive reflexives (like hans, hennes) but used when the subject is in the sentence ("Han elsker bilen sin" vs "Den er bilen hans") If hans/hennes is used while the subject is present in the sentence, then the posesive refers to another person being the owner and not the subject ("Han elsker bilen hans" = John loves Peter's car)
I believe si-sin-sitt is used when the item belongs to the subject, and hans is used when the item does not. e.g: Abby and Bridget are in the park, and Abby kisses her husband. If 'her' in this sentence means Abby's, use si-sin-sitt, meaning HER OWN. But if 'her' means Bridget's, then you would use hennes. tl;dr: si-sin-sitt = his/her own
In this case, you couldn’t use “eplet sitt”, because the apple itself is the subject of the sentence.
But if you had “He is eating his (own) apple”, you could write “Han spiser eplet sitt”.
If he’s eating some other dude’s apple, then it would be “Han spiser eplet hans”.