Honestly, I'd be horrified if my 4 year old (girl, if that makes a difference) behaved like this. My parenting is on the stricter end of the scale admittedly. I do agree that all kids behave badly from time to time (mine included), and this is obviously one of those occasions that needs firm words, an apology to your son and an appropriate punishment from your sister. I wouldn't expect your Mum to take sides, but how frustrating that she is condoning what is clearly unacceptable behaviour. Unless she thinks your nephew is somehow justified in taking whatever he pleases from others with no consequence, my opinion is that she should either stay out of it or gently reinforce the message that she expects considerate and fair behaviour from all her grandchildren. I'm not surprised you came to the forum for a second opinion!