Incest is generally frowned upon with people but is regularly practiced with snakes. Long-term inbreeding (uh, albino burmese for example) has a detrimental effect but is still regularly done. Breeding siblings to each other is the shortest path to the desired result when playing the designer morph game. Taking the time to outbreed, adding fresh blood/DNA, takes patience and time and often plays second fiddle to the financial gains to be had. That being said, snake DNA appears to be much more resistant to the nasty side effects of inbreeding (compared to humans).