I'm not a native speaker, so take this for what it's worth:
I think you would have to use the verb "saber": Ellos no habían sabido nada de su madre.
Here's a link that explains the difference between "conocer" (to know, to be acquainted with, to meet) and "saber" (to know, to know how, to know about)
http://spanish.about.com/cs/vocabulary/a/verbs_for_know.htm "Conozco a Pedro." vs "No sé nada de Pedro." ("I know Pedro" vs "I don't have any news about Pedro.")