If a male cat gets his sister pregnant will the kittens be deformed?

I recently acquired two cats, both of which I thought were female. The one I found to be female has fur so long it disguised his male appendages. I did not discover his sex until I caught him "getting down" with his sister. If she is pregnant will the kittens be deformed? SHould I go to the vet and get an abortion? Feel free to laugh at me if you wish but please provide information if you can!