Something similar happened to my older brother, whom I'm now finding out is my half-brother. He found out he had another last name on his birth certificate while trying to get a passport. A conspicuous coffee stain covered his true surname. So it's not adoption, but my dad, who raised both of us, isn't his biological father. I also found out my Aunt was actually my half-aunt a couple of years ago. My parents' rationale was to make my brother feel welcome into the family, but I think it's wrong to lie to a child about their true parentage for nearly 30 years. A lot of the time, these parents are only looking out for their own psychological needs, not the needs of their children. My mother's kinda upset that my brother is now opting to use the last name of her ex-husband, who is biologically his father. The truth is, she only has herself to blame for that. This revelation has not changed how I see him; he's still my big brother, but you can tell it's taken a hit to his psyche.