Male cat trying to mate with his own sister?

They are both around 5-7 months old, and the male just keeps "mounting" her, and biting the back of her neck, they are siblings, and its just disgusting for them to be doing this. I CANT get him nuetered, my dad suggest just dumping the male off somewere, and i really dont want that, i want him to have a loving home! what can i do to make him stop? if the female had kittens, would they be deformed?