How likely it is to create deformities greatly depends on how closely related the parents were. For example if the original mother and father were siblings, then a son of that mating also impregnates his own mother or sister the chances of defects in the resulting babies are much higher than if the original mother and father were not closely related. It also depends on the genetics of the individuals, for example if they had the roan gene then inbreeding greatly increases the risk of getting lethal white pups.
I assume the hypothetical situation we are talking about involves an incorrectly sexed pup being left with the opposite gendered family members, in which case much more concerning than the chance of defects is the risk of complication to the mother/sister. For the mother the risk of complications is greatly increases due to it being so close to when she last birthed and the fact she would still be lactating when being impregnated again. For the sister the risk of complications is also high as she would be getting pregnant at such a young age and would still be developing herself while pregnant.