oh i think the sling is a good idea to safeguard the baby, and i don't think MmeB should stop seeing her family. I'm saying that the sister needs to be on that boy like a heat seeking missile.
He'll soon stop the behaviour if he's told every single time that he must not hit. Some here are saying 'it's just a phase.' like it's ok to just let him do it cos he'll grow out of it. Er, NEWSFLASH, he'll not grow out of it if he's not told. 3 is plenty old enough to be told not to hit... It's not inevitable, and we as parents have it as our duty to show our dc that hitting a baby is seriously not on.
MmeB, how would your sister like it if a 5 or 6yo kept coming up to her ds and hitting him? Would it be enough to shrug shoulders then?
I'm guessing that, no, it wouldn't be..
At 3yo the child may not be old enough to fully understand the consequences of his actions, but he's old enough to be told that it's not acceptable.
You could try teaching your dn how to stroke your dd's feet, only the feet, and never the head... Praise him when he is gentle... Have another gentle word with your sister, get her to put herself in your shoes.
Don't pander to the boy, get his mother to teach him what is acceptable, and what is not.