I am inviting my mother-in-law on super-visa. The consulate asked her to submit the proof of relationship with stated son-in-law.
Can you please suggest me what documents should be submitted?
Actually it is your mother-law that is the applicant. Your wife would need to submit an invitation letter, co signed by you along with documents (T4, most recent notice of assessment, etc.) showing she meets the income requirement. If she doesn't you can submit yours as well and the processing office will consider your combined income.