Switched At Birth
My secret is that my grandmother was adopted—sort of. See, my great-grandmother miscarried her child. At the time (1930s/40s, I think), all women were put in the same ward in the hospital after they gave birth, even if the baby didn’t survive. Anyway, there was a woman in the bed next to my great-grandmother who had just given birth, but she didn’t want a child.
She wanted the hospital staff to “get rid of it”. So they just…gave the baby to my great-grandmother. No paperwork or anything. They had one woman who was distraught at having lost her baby, and another distraught having got her baby, and I guess they just thought “I know how to fix this”. It seems so weird by today’s standards (and definitely not legal) but I guess it was perfectly logical. As far as I know, they all lived happily ever after.