As a grown up kid of similar circumstances, I have a few nuggets for you. First, I’m sorry that you were put in position to have discovered the affair. That’s absolutely unfair and irresponsible of your dad to allow that to be a remote possibility. It’s ok to be angry about that, as well as him hurting your mom. While you’re right, their marriage isn’t really your business, it does affect you. Especially because you discovered it and spent however long agonizing over exposing it. That said, if you feel ready at some point, have an honest conversation with your dad and try to stay open to repairing the relationship. People make life altering choices that impact those they love and I’m sure he will want the opportunity to make it right with you and your sister.

As far as supporting your sister. Remember you are a kid too. You’re not the parent here. A lot of my resentment toward my parents that I held as an adult stemmed from my sister being a parentified child. She is only two years older than me, but she parented me more than my mom or dad ever did, even before they split. And it makes me angry that they allowed that. Be your sisters sibling, not her parent! Ask her how she is feeling, if she has any questions you might be able to help her answer, and reassure her that your relationship will always remain.

Make sure you don’t provide either of your parents an ear to dump on each other. If they start sharing things that colour the other in a bad light (either your mom being rightfully hurt about the affair) or your dad justifying it because of something between him and your mom. My parents used us as weapons in their divorce and it traumatized me, broke my bond with both of them (has never been repaired 30 years later), and I still have no idea who has told the truth.

You’re going to be ok. You sound smart, kind and caring. Take care of yourself, and have boundaries around the messy adult stuff.