I have 2 nine week old yorkie puppies that fight (brother & sister). How do I stop the fighting?

Hmmm….

Pretty good question.

Nine weeks old.. that means they have been with each other pretty much.

Try introducing them. Of course, this won't really help if you live in a one room house… sorry if you do.

Put up a double baby gate (buy 2 baby gates and put them about 12" apart) in a doorway and let them sniff each other through it. Then, when you think it's about time (I think that you should do it about 1 week) put them in a room together. Make sure that you put a leash on both of them! If they stay calm, walk towards a door. If they follow nicely, then they are both perfect! If not, then put them back behind the baby gates, and then just do it for another week. By then, they will have to have liked each other through bars by then.
References :

Do they fight or do they play fight? My dogs are 2 different breeds and they get along perfectly they play all the time. When they are bad punish them like children. Yell at both of them and tell them to stop.
References :

they are just play fighting. no worries, it will stop when they get older.
References :

Leave them alone! They're playing with each other like normal puppies do - figuring out where their place is in the pack and learning things like bite inhibition. Why wouldn't you call your puppy's breeder to ask questions about normal puppy behaviour? A good breeder is your best resource for common sense information.
References :

Yorkie puppies fighting. omg. I'm surprised the UN hasn't intervened.
References :

a spray bottle filled with water. it doesnt hurt them and after they start to see the bottle and have been sprayed a few times they will stop before getting sprayed.
References :