I saw that too. What I think happened is that Grace died, and that Mary legally adopted one of them. In the US, when a child is adopted, their original birth certificate is removed from the records, and a new one is issued with the adoptive parent's name on it.
Birth certificates for each year are numbered consecutively and in chronological order, you can always tell the adoptions because they are at the very end and out of order.
I can't find the middle daughter either, I'm wondering if she was adopted out too.