My cousin is adopted, but for years we were told not to mention it because she didn’t know. I was talking about it with my dad one day because I couldn’t believe she didn’t know.
During the conversation, he let it slip that my older sister was conceived using donor sperm. After a moment, he added that I was also conceived using donor sperm. Our two younger siblings, however, were not and were completely unexpected.
It turns out that my older sister knew, but that my mom made both my sister and dad promise not to tell me. I’m not entirely sure why I wasn’t supposed to know, but they managed to keep it a secret until I was 22 (I’m 23 now).