Brother and sisters are simply NOT ever allowed to mate! Would you have babies with your brother? Eww..
They will if left to their own decisions. They do not know and don't care if they are related and will mate when the female goes into heat. One of the dogs needs to be fixed (rather both should be fixed anyway if they aren't for breeding) If they were to breed, it would be a terrible incident soley due to your own irresponsible actions.
I kinda hope this was a joke question..
