What is your relationship with your mother-in-law? Do you have many feelings for her? What we see in our dreams is actually the visualization of some emotions. What image of fear did you see? If you can describe it, you may know what emotions are bothering you and your mother-in-law, and what problems between you cause you to have such nightmares.
My suggestion is to deal with the relationship between mother-in-law and daughter-in-law, and resolve each other's emotions and grievances, so that you won't have such nightmares again. If you feel that you can't do it, you should also seek the help of a psychological counselor.