Please follow the Find A Grave guidelines for requesting transfer of a memorial. That includes DIRECT relatives within four generations; i.e. siblings, parents, grandparents, great-grandparents, children, grandchildren, and great-grandchildren.
Creating memorials takes time and effort, and usually there is no need to "take possession" of a memorial in order to make changes to it. That is what the Edit tab is for. Photographs can be added without being the manager of a memorial.
I am related to many of the people whose memorials I have created, but will gladly transfer under the above guidelines if I am not related also.