To create a t-slip, you would need to register for a payroll account with the CRA, and then submit the slip(s) to them by a specific deadline. You would have also have need to have processed a payroll where you took off taxes, CPP/EI, etc... every time you paid your mother in law. So it's too late to do this now for 2019.
Should you do it in the future? Well, unless you are paying your mother in law the same as you would a non-family member for the same services, then the CRA will just see it as you giving money to a family member, which is not taxable or deductible.
