You use "sin" when referring back to the subject (in 3rd person) of the sentence. You use "hennes" when referring to the posessions of some female other than the subject of the sentence. If you had used "hennes" in this sentence, the woman would have been eating some other woman's lunch.
They're currently only available on the website, unfortunately.
When on a computer, you can find them either by clicking the skill and scrolling down past the lesson overview, or, if you have the new Crown Levels update, by clicking the skill and then the light bulb icon.