Their parents are morons. They could have just kept the mother hidden for like a year, and one of the kids for a few years. Alternating between them, so both get to go out. To avoid explaning the lack of a second pregnancy and the second kid not being a baby. Then just present them as ordinary siblings, one year apart. It's not like they were social butterflies, since they kept it hidden for so long anyway.
Still, can't expect much from a medieval-ish society. No general education, after all.