Can my mother remove a person off the deed to her house without that person needing to know?

No.  If your Mom was competent when she put Sister's name on the deed (presumably along with Mom), the Sister is the co-owner with Mom.  I will assume from the question that the deed is a "survivorship" deed, which means that Sister will become the sole owner when Mom dies.  The only way to get Sister's name off the deed is for Sister to voluntarily sign a deed to accomplish that -- or for a Court to order it.  If your Sister committed some kind of fraud in getting Mom to add her name, a Court may well order the change.  Even if Sister's name is removed from the title, there is nothing in your comments that suggests there is a means to assure the handicapped brother will have access to the house.  You should see that your Mom visits a good real estate attorney in your area to explore a means to recover the title AND to see that proper arrangements are made to preserve the title for your brother.  You mentioned that the property has a reverse mortgage.  If that is so, the lender may have a claim superior to your Sister and your Mom.  Again, a good real estate lawyer should be consulted.