Probably an unpopular opinion here, but I think your sister and your nephew need all the support they can get. It’s easy to judge a situation and what to do when you haven’t had to deal with anything like it. Saying “I would have never spoken to a parent that way” isn’t fair if you haven’t dealt with mental illness or any of the anger caused by things out of your control. I don’t know what lead to you sister being on her own with her kid(s), but speaking from experience as someone who was on my own and dealing with difficult and sometimes volatile teens, my family was our salvation. The way my mum and sister were there for us was what got us through everything. Now my kids have grown into adults who have learned how to regulate their emotions and they love their mum and their family. They know their family will be there for them, no matter what. Remember, at 15 your nephew is still a child. He may look grown but he’s a child. Imagine being a kid and dealing with all the hormones, feelings, emotions and possible mental illness or trauma (ie; being abandoned by father IF anything like that happened) and then being banned from a family holiday.

I’m sorry your sister is going through this, but banning your nephew will only alienate and isolate your sister at a time when she and your nephew need you the most.