Is Mandy altered? Do you know if she's previously been bred? I do agree that she's trying to be alpha as hard as possible, and Molly is reacting as well trying to maintain her "babysitter" role. Molly having had pups may either believe that she's now alpha or at least very close under her human parents.
The only thing that I would try is to have them both leashed and purposely allow them to see each other, from a 5-10 ft distance. If either of them show body language of dominance or agression it can be corrected quickly as you have full control with the leash.