What I would do is, while waiting for the females to grow up, mate their mother to a male that is not their father, (or mate their father to an unrelated female) creating (hopefully male) half-siblings that are a year younger. Those male half-siblings would be their future mates. That would ensure a good chance that the recessive gene is present in both of the half-siblings while still introducing new genes from an unrelated snake.

when outcrossing like this, i would prefer to use a WC snake so you know for sure there is no relation.