Anyone can be your sponsor as long as you can prove the connection or relationship between both of you. The closer the relationship or bond- the easier it would be to justify why they would be sponsoring you. As long as you can justify your relationship with a sponsor with strong evidence, you are good to go. It is not advisable to get statements from a person you have not met or those whom you are not in regular contact with. The best people for sponsorship are immediate family members (father, mother, sister, brother).