Work based conflict

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ok, where to begin, I currently work in a village shop, its owned by a nationwide company and is more of a small supermarket than a newsagents, but back to the story, the boss has had a habit of over the years giving her children jobs within the shop to keep them out of mischief etc. Currently we have the joys of her 18 year old son working there, he's also the youngest member of staff by apx 6 years and I have never in all my work experience met somebody as rude and immature as he is. he's never on time, never wants to do any work, believes that the rules in the handbook don't apply to him etc now I have let him get away with a lot over the past few months but recently he has got worse and worse, I have just spent a week on sick leave with tonsilitus where he had the nerve to broadcast round the shop I was faking it despite having a letter from my doctor and taking prescription only medication to get over it, I then subsequently caught him (today in fact) out in the stock room, sitting on a chair that he had made out of items of stock, listening to his music and not having a care in the world, where as I was running around trying to keep the shop running and the customer's happy. My question is what do I do? do I report him even though he is the boss's son, or do I let it all slide yet again?