Families almost always have some secrets. When my mom died in 2016 (at the age of 86), a cousin of my father told us that my father had a child before he got married to my mom. They approached my dad's sister back then, when the girl turned 16, in order to tell her who her father was. But my aunt (whom I never got to know) said that this won't happen and our cousin told us that my dad would not acknowledge this child of his. So, no word got out until more than 50 years later. Nobody knows if this woman is still alive or not. By now, the elders are gone and we cannot ask anyone.

I wonder, do you have contact to this half sister of yours? Ever got to know her? May I ask how old you were when you finally found out about this secret?
At least she got a present from her dad and you did so, too.

Life is always very interesting :)