Fall out over 8 year nephew bad behaviour

[40nanny]

Long story,but just had fall out with sis in law over sons bad behaviour,anytime he comes,it ends up in a fight with my sons crying,pushed my 4year old into wall,turns telly off,cod he didn't come to watch telly,I've tried playing snap,football etc no doesn't like that,he takes almighty tantrums over basically silly things,now I know this is not normal behaviour for that age,if this was my sons friend he would never be allowed back in my house,but coz its family Ive bit my lip for years,but couldn't stand seen my kids being annoyed& in tears,so finally said something,& now we had a fall out & she not bringing him back,I'm actually glad cos my boys don't even like him,now I'm the bad one,but I couldn't put up with it,kept thinking he will get better.but didn't think it would end like this.feeling bad now.what do I do?