What do you do when your dog attacks its (mother dog)?

If it's a continuous problem then you may want to consider keeping them separated or getting rid of one. You may try some obedience training.

The most useful thing I have found that works on an aggressive dog is a shock collar. Praise your dog when it is playing or being nice to the other dogs, but when it starts getting aggressive, give it a little shock. Also if it is a male dog, think about getting him neutered, that is very helpful also.

Do not use a shock collar if you are not trained in how to use it properly! It can do more harm than good if used incorrectly.

Carefully observe BOTH dogs. I have seen my guy attack him mate but it was instegated by the mate giving him "the eye." If a dog thinks he is about to ge attacked he might just do his own attack. Shock or E collars do work but you must know what you are doing. The dog must be shocked or stimulated the instant the behavior happens, waiting even 30 seconds and you will mess his head.