If you want to select for particular traits and improve on your flock, then you are best off using somewhat related birds -- it is ok to start with brother/sister if that's what you have available, but you will have to see how it goes. You may have smooth sailing for a good while, or you may run into problems (with fertility/hatchability as much as anything) in a generation or two, depending on the particular stock you're starting with.
However if you are not really trying to "do" anything in particular with your flock, just sort of propagate it so you always have purty chickens in the yard, it is probably easier to start with fairly-well unrelated stock, like bring in an outside rooster. And possibly repeat the injection of an unrelated rooster every few generations. This will make it hard to select for desirable traits BUT will also make it much less likely that UNdesirable traits (deformities, severe lack of type, poor fertility/hatchability) will rear their ugly heads either.
Good luck, have fun,
Pat
