My DH is a Julian and I tease him I often call him Julie or Julio or lots of other names to be honest and not all of them good ones
He gets called Julz alot by his mates but even he said that the alot of the Asian guys at his work all call him Julie (sounds like Juri)

I would tell the nephews straight out that its not on. I would expect my siblings to tell my kids off if they deserved it and especially if they are bringing out your "mother lion" I would definately let them have it.