It's actually going to be easier now that all hope is gone. I know it hurts now but that hurt won't last forever. You have been through this before and you got through it... But no more after this ok? You need some peace
This is on him, not you. Anyone capable of doing this isn't worth it. Let her have him. He is not a prize. He is a liar and he is disloyal and he is selfish.
I am sorry for your step daughter though. You are right to not get her involved. But you can't make someone do the right thing. She is lucky to have you as a positive influence.