Omg I feel for you. It’s very unhealthy to involve a child (of any age) in parents‘ disputes and discords. I wonder if you could change topic of a conversation every single time or simply end a conversation like she starts talking about dad, mom I gotta go. If she needs to vent she could do it with her friends or she might benefit from therapy. It sounds so frustrating. Sorry you are in this situation!!!

What she is doing isn’t normal

What you are doing isn’t selfish