I would have to say no also. My ex had three cats. The mother, and then a brother and sister cat. Now, this brother and sister were only "half" brother and sister (came from two different litters). Well, neither were fixed, and well, the bro and sis mated. One of the kittens had Tourettes Syndrome. Not only did it twitch (blinking tics), it would meow incorrectly...it would go..."meo meo meooooow." The kitten was also VERY small, and more or less stayed kitten size. I would go with a different breeder. Oh, and nice job on asking first!
Also, did you do breeder research first? We just unfortunately had a member on here who wanted to breed her female. She was her beloved pet. Well, her female died, and so did all but one of the pups. She said she would have never allowed her pet to get pregnant if she knew this was a risk. I was thinking of getting my female pregnant before joining YT, but not after everything involved with breeding and the risks. NO WAY. If it's one thing that the members on Yorkie Talk taught me is to do LOTS of research first before making a decision. And as I said above, good for you for asking first!