My personal theory is actually that the Whamp females are infertile. We've never seen any mention of any children of a White Vampire Woman. Vittorio is described once as Cheserea's son, but in several other places as his aunt.
In any event, I think Mab sees Lara as a potential replacement for Mab. This is why she wants Harry to take out Molly if something happens to Mab. Molly is nowhere near ready to be Mab. Lara, on the other hand, has shown herself capable of the ruthless pragmatism and icy cold logic that Mab sees as her mode of operation. Moreover, Lara has through skill, manipulation and personal power survived for centuries and shown herself to be the strongest of the White Court -- not a negligible accomplishment.