Not Actually the Oldest
There are some things that we wish we never knew. For instance, people never want to learn that their moms and dads are swingers and hook up with the neighbors next door, especially if you dig their daughter. This story made us quite sad when we first read it.
The OP goes into a full-length story about what happened exactly, but in a nutshell, they learned that they had an older sibling. The OP was the eldest child, at least as far as they knew, but they learned from looking at their family pedigree and after their mom had a mental breakdown that their older sibling died in the womb.