AIP (for Inland applications) is supposed to be based on:
Sponsor approval
+
Assessment of applicant's relationship with their sponsor.
This is detailed on the flowchart in ip08 (page 43) and clearly shows that the first item (or question #4) AFTER sponsor approval, asks if the FN is a member of the [Family] class and asks if they are a spouse or common-law partner of a sponsor. Once that has been established, along with 4 other items that need to be assessed, then the AIP is issued.
If they are asking for more proof of your relationship, how could they have issued AIP...unless that is yet another change in how CIC processes applications now!
AIP is supposed to be a letter labeled Appendix B, and begins with:
This letter acknowledges receipt of your application for permanent resident status in Canada.
It has been determined that you meet the eligibility requirements to apply for permanent resident
status as a member of the spouse or common-law partner in Canada class. However, a final
decision will not be made until all remaining requirements for becoming a permanent resident
have been met. These requirements include medical, security and background checks for you
and, if applicable, all of your family members, both in Canada and abroad, even if they are not
applying to join you in Canada at this time. You cannot become a permanent resident until you
and all your family members have met these requirements.
Is that the letter that you received?