Breeding siblings is not always recommended, but is done regularly. I, myself, have bred siblings with much success. I believe that if they are the first generation to breed together then all should be fine. Deformities and other problems will occur if continued inbreeding takes place generation after generation. On another note, at two years of age a Mexicana complex snake (although alterna are not considered in the complex anymore)may not breed. It usually takes three years for them to mature before breeding. I usually wait until the female has had a levelling off in her growth rate then attempt hibernation and breeding. 80% of all my Thayeri, Greeri, Ruthveni, Alterna and Mexicana Mexicana usually breed thier third year. Seldom do I have luck the second year. Others may have, but not I. If you feed more often than weekly then you may have better luck, but I do not recommend breeding an undersized female. It could cause deformities and other problems. It can also stunt the female's growth (I am told).
Good Luck and have fun breeding. Waiting is the hardest part.
John Lassiter