The word "possessive" might be a bit misleading. Yes, we use possessives for things that we actually possess, but we also use them for relationships. When I say "my sister" it's not because I own her, but because I'm defining the relationship between us. Here are some resources for you to explore this topic.
Online activities from the Exprésate textbook
Book 1 Chapter 5A p. 162 (Possessive adjectives)
Book 2 Chapter 5A p. 174 (Possessive pronouns)
Detailed grammar explanations with self-correcting exercises
Excellent grammar explanations with video
Music videos to help you learn!