I am not finding anything that specifically states that this is not allowed, unless you are claiming a caregiver amount for your mother in law, infirm dependant credits etc.  Bear in mind that paying her will consist of wages, for which CRA will expect you to withhold and remit source deductions.  You will also need to provide annual T4's to report her income as the employer.  Other people with more information on this topic may check in hopefully.