But I think Mrs. Pompom would have noticed that. For Mr. Verres to be so certain, the birth records would have to indicate Mrs. Pompom is Susan's biological mother, and Mrs. Pompom would most certainly know that Susan was her daughter (unless there was a mix-up at birth, which is a rather dark turn to take).
But one thing that has to be noted is that Susan shares her hair color with neither her father or Mrs. Pompom. And Jerry said that Diane was Susan's long-lost sister. Unless there is something Mr. Verres missed (depending on how you interpret "long-lost sister"), shouldn't have Jerry been more specific?
That said, Verres only checked Susan's record, so we might see things cleared up a bit more if he dug around Diane's birth record. But if Mr. Pompom is parent to both Diane and Susan, that's a very bloody big coincidence for both children to be born within twenty minutes of one another, and grow up looking like twins with a hair color they couldn't have inherited from dear old Dad.
All we know right now is that Mr. Verres indirectly confirmed that Diane is the only one who was adopted.