Mating offspring back to their parents is called line breeding and is a very common practice. At some point you would want to add new blood, but I think you could several years with just line breeding. The breeder I got my flock from said "when they start shrinking you've gone too far". :lol...
I don't think that will be much of a problem. From what I have read and been told, you can safely breed full siblings for 2-3 generations before problems arise. With yours being half siblings is even better.