In most animals, that live social with each other (like cats and dogs) the nature added a built-in-health-system.
One part of this is, that individuals that are relatives should not have children together. (Children that get the same genetic material from mom and dad have a very high risk for illnesses).
In most cases this incest will be avoided by chasing the children off, when they are old enough to live alone. This behavior is nature-given (innate) to the parents and could not be trained off (as far as I know). In some cases the nature only wants to chase away the male children, in others both (male and female children).
Under these circumstances it could cause a lots of stress and aggression to the old dog and its child, if they need to stay close together.