Yes 8 months is too young to consider breeding a dog because the female dog at this age is still growing and developing her own body and now the energy devoted to that will be going into puppies instead. In addition many really young females are not good mothers and can have a hard time raising a litter.
It is not considered a good idea to breed brother and sister. This is called inbreeding and most people would avoid doing this.
You could double up on unsuspected recessive traits and have problems in the pups.
You may also get a smaller than normal litter if some of the fetuses don't make it to being born due to defects.
You might want to read more about inbreeding and other breeding methods to understand the risks.
You can read about the pluses and minuses of inbreeding here
www.dogbreedinfo.com/inbreeding.htm
http://www.sloughi-international.com/articles/linebreeding.html
http://www.geocities.com/farmcollie1/inbreeding.html
http://omalmalamutes.com/omal/breedingmethods.htm
Plus breeding two chorkies doesn't get you more chorkies. You only get a cross breed dog by breeding two dogs of different breeds.
If you saw your dog tie (mate) with a male its very likely she is pregnant.
This site goes over signs your dog is pregnant
http://www.debbiejensen.com/signs_of_pregnant.html
You can ask your vet to run a relaxin test to see if the dog is pregnant too
http://www.bullwrinkle.com/ShoppingPages/dog-pregnancy-test-canine.htm
Dogs are pregnant for about 58-65 days, or about 9 weeks counting from the first mating.
You can read more about pregnancy and whelping in dogs here
http://lowchensaustralia.com/breeding/pregnancy.htm
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics