The Lying rule applies, but it's a bit redundant as the Mothers are required by cosmic law to speak as vaguely as possible with multiple meanings, just look at Mother Summer's conversation with Harry on the Wall to the Gates.
Mother Summer is talking about Maeve, Mab, and Harry's ability to make their own decisions despite their mantle's influence at the same time. As well as giving hints as to the fact that Mab is more human in caring for her daughter than she let's on.
||The Lying Rule Theory||
I have a theory, humor me for a minute. What if the Mother's are not Bound by the Lying Rule
persay, rather they are bound by the same Root Cause of the No Lying Mandate, which is a Law against infringing on Mortal Free Will, and is the same root cause of the No Killing Un-Affiliated Mortals Law and their whole Bargains/Letter-of-the-Law obsession. What if they are limited in how they are allowed to interact with and/or restrict Mortal Free Will in the same way Angel's are. But lacking the knowledge and perception of an Angel, the average fae gets a Letter of the Law mandate (Never Ly), whereas the Mothers are held to the more direct/real/cosmic restriction of actual Free Will Interference, which they can deal with because they can actually percieve the Free Will driven branching multiverse. Like how they werent allowed to mention the Unicorn until asked about it, or conversely how they can have a conversation while still knowing everything the other knows, and possibly even being fragments of the same Entity. Thoughts?