NTA

If I were in your shoes, I would sit down with your stepdad and let him know that you were really young when this all happened. Tell him that now that you’re older that you realize the reality of your mother’s actions and you understand now what she did to him. Let him know that this has been taking a toll on you and that you care about him and feel like he needs to know the truth.

She put you in the middle of this situation and it doesn’t sound like she has any intention of telling him.

I hate that people do this to the people they love. She essentially left him, sold her engagement ring to fund her affair, lied to her husband, and drug her child along with her through all of it. You and your stepdad both deserve better treatment than this. It’s not fair she put you through this and you sound like you desperately need to get this off your chest.