We had Sierra for 9 months before we got Buddy. Sierra would jump up and bite us on the arms and nip us on the butts, and run away (she thought she was playing) it started to get BAD, then we got Buddy. They play and Sierra leaves us alone (for the most part )

I think if you get his litter mate, he may stop nipping your son.

As for the training, what one does, chances are the other will do the same!! It's like playing follow the leader!
At least that's how it was in my house! Sierra was 5 months when we got her, and she was 14 months when we got Buddy and Buddy was 1 when we got him.
I don't know how old your pup and his sister are?
But the younger the better!! Less bad habits to break!!!