Mental Retardation is not a disease, it is developmental problem. It is from birth and cannot be treated. Her parents must be aware about her mental status and still got her married to you, without informing. You have to accept the way she is, as there is no cure. If you cannot, then you may seek divorce. Mental Retardation is a direct ground for divorce and you can get it very easily. Her parents may bully you for avoiding responsibility but you have to act assertively.