I'm sorry that your question wasn't answered in a timely manner. It appears that Sapphire has "taken ownership" of her litter tray to the detriment of her grooming and her housemate. The most expedient manner in which to address this problem is to add two more litter trays in easily accessible, unlikely to be disturbed areas. Having one more tray than cats is prudent.
Her "compulsive grooming" suggests psychogenic alopecia - a neurotic over-grooming - but we have come to realize that most of these cats are allergic cats. Feline psychogenic alopecia is overdiagnosed but does exist. Excessive and out-of-context grooming is thought to be an obsessive-compulsive behavior that's triggered by environmental stresses and anxiety (which may have precipitated her litter tray behavior as well). The condition is uncommon in cats, with purebred cats that have high-strung nervous temperaments being possibly predisposed.
This type of alopecia may occur anywhere on the body where the cat can lick but it most commonly involves the medial forelegs, inner thighs, perineum, and ventral abdomen. Hair loss is often bilaterally symmetrical but remaining hairs don't epilate easily. The top differentials are flea allergy dermatitis, food allergy, dermatophytosis (fungal infection), other ectoparasites (mange) and atopy (allergies to environmental allergens such as pollens, molds, dust, and dust mites, etc.).
The underlying cause of the psychological stress (e.g., separation from owner, moved to a new house, animal companion died, new pet in household, formerly outdoor cat denied access to outdoors) must be identified and appropriate environmental modifications made, if possible. A good flea control program should be instituted to prevent fleas from aggravating the symptoms. Use of a mechanical barrier (e.g. Elizabethan collar, T-shirt) for 1-2 months to prevent grooming may help break the habit. Behavior-modifying drugs may help stop the abnormal grooming behavior. In some cases, treatment may be discontinued after 30-60 days of therapy; in others, lifelong therapy is required for control. Drugs that may be effective include the following: amitriptyline, clomipramine, buspirone, phenobarbital, diazepam, and naloxone.
Please respond with further questions or concerns if you wish.