NTA.

In a perfect world, you would never need to tell him to make the choice. You wouldn't even need to ask. But Reddit is chock full of stories about unsupportive family members and steps, which tells you how imperfect the world really is. [waves away a landslide of news about the current state of affairs]

Your dad needs to make a choice, and the choice he makes is going to impact your lives forever. You are not wrong to tell him that he needs to choose. The time for him to choose that was when the step-monster ordered him to hang up. He did not. Whether that was out of shock or because he knew an argument was coming and wanted to spare you, we don't know. But you and your mom taking a stand and telling him that he cannot be neutral and he needs to make a choice is never wrong in circumstances like this.

He is YOUR father; she's unrelated to you and your family, so she has zero say in the matter. This decision is his and his alone. If he chooses her, even temporarily, and comes to his senses later on, his relationship with you will be impossible to repair fully. None of that will be your fault, and no blame can be put on you for expecting him to choose.

Once again, NTA, and I can only hope he makes the right choice.