I don't have the game yet but does this have multiple endings?
I'm watching someone play and early on I just saw a scene whereMia's brother got turned to ash but I thought there was pics of him in the main base/hub in promotional materials? So is there a way to prevent that?
Yes, there are multiple endings.
When you defeat a successor, you can choose to awaken their memories. They will stay alive if you do this, but still in their transformed form. However, if you simply walk through the gate in their memory, they will be gone for good.