Are you referring to the random chance that someone is irresponsibly breeding petstore chins and the chins just happen to be related, or that if put in the same cage they will breed?
Sheri is right, chins, actually animals in general, don't hold the social restrictions about breeding that humans do. Actually some humans don't even, if I recall correctly some peoples still interbreed. Aristocrats used to marry cousins, and even sister/brother to "keep the blood pure".