People fall short, you have a great dad but doesnt mean he dowsnt make a human mistake. Now this mistake can grow up and consume a lot, you dont need your mom to find out, butbyou need to calmly tell your dad that you know and that it's not a good idea! Not in a scolding way, just as if u are expressing ur opinion, dad, i know tgis and it's not a thing for the welfare of the family. Dont make it look like a threat or blackmail.