Why would a mom say her twenty year old son and his younger sister call her and their dad mommy and daddy when they don't especially her son?

Answer 1

Dunno, ask her (that's almost always the, strictly speaking, correct answer for this kind of question). However, if I were forced to guess, it's because she wishes they would do so. It's not all that uncommon for parents of kids this age... particularly mothers... to try to extend the "my darling little baby" phase, even when any reasonably objective observer would conclude that ship has long since sailed.