He looks all yorkie to me except that he'll likely be closer to the 15lb range as an adult. I have seen pups prior to the colour breaking at 12-14 weeks with the same kind of snout, face, down ears, etc, who came from 8-10 lb parents who appeared perfectly proportioned like what the standard Yorkshire Terrier looked like back when they were a larger breed. The offsprings may very well grow to be larger than their mom and dad.
If your girl is within the AKC breed standard of 7 lbs and under, I'd be concerned about pairing the two as that pup, if he hasn't already outgrown your girl, he will get twice as big especially if he is still just 12 weeks young now. That being the case, his offspring will likely get as big boned and I'd worry about the safety of your little girl carrying the pups to term. And this is with the assumption that your brother's pup didn't just happen to be the SMALLEST one of his litter so then, he could have even bigger genes to pass on.
If you MUST breed, please find yourself one, or ask your brother to do so, a girl who will be guaranteed to be larger than his male pup as adults. That way you know her line were all 15+ lb'ers and wouldn't need to worry as much about your brother's male throwing offsprings too large for the mama's uterus to carry safely and to deliver healthy. Also, from the ones I've seen and also read, the larger yorkies tend to give you 6-9 puppies in one litter!
That's a whole lot of dog and potty accidents to clean up.