What is OCD?

Your mother-in-law sounds like my mother-in-law too. There's a difference between being a perfectionist and having obsessive-compulsive disorder. Perhaps you keep the floors in your house so clean that you could eat off them. Or you like your knickknacks arranged just so. That doesn't necessarily mean that you have obsessive-compulsive disorder.
In OCD, your quality of life can decrease dramatically as the condition dictates most of your days and you become consumed with carrying out compulsive behaviors and rituals. Left untreated, obsessive-compulsive disorder can make your life unpleasant and unenjoyable. You may feel that you're being held hostage by OCD because you're unable to go about your normal routine and activities.
Your doctor is the only one who knows what treatment is best for your mother-in-law. The doctor will put her either into Psychotherapy or medications.