The mistress tricked me into a friendship

The title says it all. The kicker is: this was the “other woman” in my parent’s relationship, not mine. She used to work for dad and so did I when I was freshly out of college. She slowly became my friend, gained my trust, would confide in me about her dating life. I once saw a “moment” between the two of them but brushed it off (it was literally just the way she looked at him once). Shes from a culture where arranged marriages are the norm and virginity is a must. Never in a million years did I even suspect it. Not only did she become my friend but she was a “spy” and was reporting back to my dad every detail of my own dating life. He was also trying to gain influence via her. She eventually infiltrated our family and spent many holidays with us. She even called my mom “mom” and became a surrogate daughter (her own parents live overseas) We trusted her.

We found out a good ten years down the road when my mom found sexts. Dad begged mom not to tell my siblings and I but the betrayal was too much. She begged us not to tell him we know. I’m disgusted. Mom has decided to remain married to him so I’m not quite sure why she even told us about it then. I’m having a wedding ceremony after this whole pandemic thing is over and the thought of him walking me down the isle makes my stomach churn. How do I proceed guys? Dad thinks what goes on in his marriage is none of our business anyway but didn’t he cross several lines by making sure she was a part of the family?? How can I ever look at him the same?

Also, the “friend” ended up showing her true colors when the jig was up. My mom confronted her and a blowup ensued. She threatened to spread lies to destroy my career (as well as my siblings), and made some jokes about mom’s recent cancer diagnosis (claimed her and dad often laughed about it behind her back). My so called friend blocked me shortly after so I suspect in some capacity dad knows we know...otherwise why no contact all of a sudden? doesn’t seem to care and will not publicly address or apologize for it. How can I even move forward.