I knew Squirrelflight was related to Brambleclaw in some way, but I didn't know that Ashfur was related. That's what happens with clan cats. A clan can't sustain itself unless it takes in new, unrelated cats. If you think about it, if a clan kept having mates within the clan then the cats would all end up being related after a surprisingly short time considering the longevity and population of Thunderclan.