Parental Problems
I convinced my (older) brother that our father wasn’t really his father, rather, his father was really my mom’s first husband. My Mom had met my Dad before her first divorce was final, so this was somewhat plausible.
I told him that she told me this and made me promise not to tell him, but that I felt he had a right to know. He believed this for years apparently.
I had forgotten about it, until my brother was 16 and got in a big fight with our Dad, and yelled, “You aren’t even my real father. I found my real father, and I’m going to move in with him!” Apparently, he had actually contacted this guy, and had convinced him that he was actually his father. I got in a lot of trouble. Story credit: Reddit / I_make_milk