My Rottie was like this. & My sister has a rottie-lad mix, hes 6 months old & as big as my other rottie(I had two, only one now).

This pup will run by my nephew (hes 5), run by him & bite his elbow. I told him when ever he does that to hit the pup with his elbow & tell him "NO"!

I believe, you, your sister & anyone else who the dog might be in contact with the pup on a regular basiest should "ruff house" with the dog for a little while each day, dominate him. Flip him over make him submissive. & when YOU ARE DONE, let him know it....... I useto tell my rottie when we where done playing. " ok we're done, BE nice" & lay him down on his back & petted him.

Worked for me.

Good luck.
& remember rotties are a little different then some dogs.