I (29 female) recently found out my father has an ongoing affair for the past 19 yrs. For context my parents have been married for 36yrs I have two sisters (35&33) and we are a very close knit family. I still spend almost weekends with my parents and sisters as we all stay at the same campground. My father works out of town and recently got a new phone. He left for work and my mom had a hunch and found the old phone with all of the contents still in it. Upon investigating we found out he has been having an affair with the same woman for 19 yrs. The woman is married and has 3children. My dad claims one of the children(18yr old female) is his. My mom called the lady’s husband and told him everything. We live in a very small town and we are not being quiet about the matter. We have heard that the mistress and her husband plan to keep it on the DL and are not informing the daughter that is not her real dad. My sisters and I feel it’s not their decision to keep this information from her as she is an adult now and away at college. We have tried to speak to the woman and the husband about the issue as we all have a lot of questions since it seems like our entire lives have been a lie. They will not respond to any of us. My father says to do what we want with the information and if we want to tell the girl she is our sister we should. My mom has filed for divorce and is not speaking to any involved parties except us daughters. AITA if I tell the girl her father isn’t actually her father and she has three sisters(and nieces and nephews) she doesn’t know about. Lies are how this mess got started and I do not feel it’s right to try to cover it up with more lies. What the girl plans to do with the information is all on her whether she wants to meet us or continue on like nothing happened that is fine but I still feel like she should at least be made aware of the truth.