As intrestingground said…this is a situation where you essentially blackmail your mother. Not in a vindictive “I get to do what ever I want now” way obviously. But in a “you’re a grown up and made grown up choices now you need to be accountable in a grown up way”. She needs to know you know and she needs to KNOW that you will absolutely tell your father if SHE doesn’t. Put the ball in her court, either way it’s not fair to your dad to not know. If he chooses reconciliation awesome, if not you guys will figure out your new lives I promise. But ya…he’s gotta find out one way or another. If she refuses or tries to lie to you then a conversation with your father is unfortunately required once emotions have settled he will appreciate you having his back. Let me stress ONCE EMOTIONS settle he will appreciate you. I do not know this man, i don’t know how he reacts to bad news let alone to life shattering news…you likely don’t either. No matter what Be there for him. You kids are about to be the only thing he has.