Dead In The Morning

Imagine an old lady who was hated because of her arrogance and cruelty towards her children, and whose lives she dominated, becoming a murder victim. But it is the housekeeper who is found dead. Had a mistake been made and the wrong woman killed? Dr. Patrick Grant uses his powers of logic and deduction to determine this is not the case, but he can only prove it at the expense of incriminating an innocent person. How does he solve this particular conundrum?