What should I include in my family tree?

I made a project for a competition abut old photos with family members and some events related to them. I came out with ten short stories about my relatives from my mother's side, and now I have to build a family tree and I am really confused on what should I include. It doesn't have to be complete or anything, I want it to show rather what connections there are between the persons in the stories. Should I include siblings of my grandmother, for example, if they carry no real importance for the people in the stories?