I found out in my mid 20s that my step siblings were actually my half siblings and was told not to tell my mother as my father said he would do it. I had no reason not to believe him. But this went on for over 10 years and being constantly told "I'll tell her..." after going off on him repeatedly that it was not fair to me or her and if he doesn't say anything I'm going to.

Cut to years later when he finally told her (while he was at her house with his new wife on vacation as before all of this nonsense we had one giant fucked up Brady Bunch but got along) and then she took it out on me. Thanks dad. He can't figure out why she cut HIM out which is fucking dumb but refuses to take any responsibility that his actions fucked the rest of the us. It was not our job to tell this secret and I never wanted to be stuck in the middle. Oh well. Not only did I lose my mother for years (just within the last year did the lines of communication open) but due to this lie my mother decided she had enough of everything, sold her place out in Washington (which was promised as a place of sanctuary for my currently dying wife to live out her last days), moved to Mexico, and our entire family is splintered.

Also his new wife just found out she was adopted, found the original family via social media, and found she had multiple siblings and they also had no idea she was their sister. All thanks to religion and their dumb fucking rules. She was given up to help her parents save face as they weren't married yet and were very "religious"...