Yes they can, and they shouldn't. I don't know how old these littermates are but a female reaches sexual maturity within 6 months, sometimes earlier, depending on the breed, and a male reaches full maturity between 9-12 months, but again, sometimes even earlier.
They will breed- it is called in-breeding and it is not a good idea. Follow this link to see what happens when unchecked in-breeding occurs. I urge you that if your litter of kittens are 5 months old or older that you get them spayed and neutered
Cats don't differentiate each other as siblings or family members. They have the drive to mate, and if a sibling is available that's who they will mate with. It doesn't matter to the cat if they are related or not. When a female goes into heat she will mate with whatever male is there, a male will mate with any female who is receptive to him. They don't have the "morals" that we humans have regarding incest and inbreeding.
A couple of years ago, my son visited a local farm where the barn cats were allowed to mate unchecked. My son said a lot of in-breeding was occuring, and many of the kittens had deformities involving their heads and faces.
I agree with Hissy, in-breeding isn't a good idea and should never be allowed
Asim started to try and mate with his sister when he was 4 months old (they both were 4 months). It was a big no no no! We got them box fixed.
Asim is already a polydactil, which I've also read that is caused by inbreeding, another genetic mutation.