In the other, an adopted friend tried unsuccessfully to trace her birth family. She never learned the identities of the actual people, but she did learn from the adoption agency’s records that she had biological siblings who were not surrendered when she was – and that her biological mother had ALSO been surrendered for adoption by parents who had other children. A disturbing family heritage, to say the least.

https://en.wikipedia.org/wiki/Twinsters

I don’t know the answer, but I have heard of it happening.

Sometimes the parent has some changed circumstances. These sisters are very close in age. Perhaps the mother gave one away but thought (later on?) with the second one that she might someday be able to keep her or get her back, and therefore she only was in foster care?

I read that story about the 97-year-olds. So sad! Although apparently they both had a good long life, but a dreadful way to go.

They were fraternal twins, by the way. The girls in this story are not twins of any type, they are sisters, but they certainly do resemble each other.