Does cats (brother and sister) have kittens with eachather

Yes. For this reason, please get the cats neutered by the time they are five months old.

Yes, they most certainly can and what's worse is that incest runs a higher risk in genetic deformities. I agree with Wonder, get them fixed as soon as they are of age.

Thanks.I am asking just because today I give away 2 kittens brother and sister and they will stay together.
Thanks.

Yes but they can be deformed and die or suffer like humans.