When informing Stef and Lena about Callie and Jude's past, the social worker tells them Callie and Jude's mother passed away when Callie was ten years old. He proceeds to say that Callie and Jude's father "obviously wasn't in the picture" and hence, they were placed in the foster care system. Later in the series we find out that their father was in the picture and was in fact the cause of their mother's death. this would've been public record and not only would the social worker know, but Stef and Lena would easily be able to find out, especially since Stef is a police officer.