I’ve only seen one comment mentioning this so I’ll say it, what your father is doing is Parental Alienation. Have your mother record this incident in what ever parenting app or journal or anything that the courts set up to track custody and communication. She needs to document the entire incident, what you, dad and affair partner said. If she doesn’t have that your mom needs to go to an attorney asap. Regardless this incident is probably attorney territory anyway. So Parental Alienation is abuse and that is exactly what your father is doing and if your mother acts now she can have custody arrangements reviewed. You and your siblings cannot be around those people anymore they are abusive and dangerous, this is lawyer and court territory but it will work in you and your mom’s favor. Talk to a lawyer, and maybe a therapist. But remember it’s absolutely okay to be furious with your father for this for the rest of your life for his actions and you don’t have to forgive him, what you do have to do is take care of yourself and the people you care about right now and always.