Let's get one thing straight; we are talking about cats.
Stray and feral cats on top of that!
They may be hyper intelligent creatures, but not only do they not speak to humans, they can't conduct genetic research, therefore, do not understand the concept of incest.
I know we've all seen the complaints of Ashfur being in love with Squirrelflight because he's her uncle, but again. They're cats. There is not understanding that inbreeding *can lead to genetic mutations and death along the blood lines.
What most people don't bother to check up on is that while Bramblestar is not her uncle, he is still her cousin. If Hollyleaf, Lionblaze and Jayfeather had been theirs, they would have still been inbred.
And while we're looking at Jayfeather, let's look at his blindness. The Erins themselves have stated that he is blind due to genetic problems, despite a wonderful story here on FF which theories that Hollykit and Lionkit actually were Squirellflights while Jaykit was the only son of Leafpool and Crowfeather, also born with normal vision. It goes on to say that as Leafpool helped her sister give birth, Jaykit noticed Leafpool licking one of the other kits and began to move forward towards his mother, thinking "I want licks too!" As he does so, Starclan basically punishes the kit by taking away his sight, and loosing the memory of the voices and the normal vision to growing up.
Now, what's to say Jayfeather didn't have inbreeding in his bloodlines that caused his blindness instead of some random mutation? What if Sandstorms mother and father had been related? What if Crowfeathers' were?
I also want to point out the four siblings Cinderpelt, Brightheart, Thornclaw and Brackenfur. We do not know for sure who there father is, but it is stated it may be Whitestorm or Lionheart, which leaves the questions raised that Bravkenfurs kits, aka Cinderheart and Poppyfrost, (the only two to have kits,) could very well be inbred considering Whitestorm was Sorreltails father as well.
*can meaning it doesn't always, considering probably 50% of the real cats in the world living today have incest in their close family.
**this is all speculation. None of it is meant to be taken as cannon.
***i do not own the story I mentioned. I do not know the author and I hope he/she doesn't mind that I mentioned it.