We now have eight Yorkies
Each time we add a new one, it just takes a little time for them to sort out their place in the "pack". I once had two females who were not happy with each other, but eventually we were able to sort it out and now it is okay. It sounds like Tiny just needs mom's assurance that he hasn't been replaced and that he is still number one. Yorkies like to be number one, regardless of what other dogs are in the house.
Good luck!
Katelin