Breeding two closely related animals together improves your chances of getting traits present in them to be passed onto the offspring. This means both good and bad genes.
You won't get deformity being any more likely, if there are not genes for that deformity in either of the sakes you breed together. You also won't have good traits any more likely, if those traits are not already present in the snakes you breed together.
Breeding animals to their siblings or back to a parent is very common to try to fix desirable traits in a line of that animal.