Maria Thins obviously understood Vermeer's art more than his wife did. Why was this the case? Did she possibly share Griet's talents?

Yes: both Griet and Maria Thins have an appreciation of art, especially Vermeer's, that his own wife Catharina clearly lacks.

Catharina is physically clumsy and is not allowed in her husband's studio unless he invites her. She wants him to paint faster so that they can earn more money, but Vermeer takes his time, devoting months on end to making a painting perfect. Catharina lacks any detailed knowledge about how light and shadow affect painting:...

Yes: both Griet and Maria Thins have an appreciation of art, especially Vermeer's, that his own wife Catharina clearly lacks.

Catharina is physically clumsy and is not allowed in her husband's studio unless he invites her. She wants him to paint faster so that they can earn more money, but Vermeer takes his time, devoting months on end to making a painting perfect. Catharina lacks any detailed knowledge about how light and shadow affect painting: for example, when Griet asks Catharina if it's okay to wash the windows in the studio, Catharina doesn't even understand what Griet is talking about when she ads that doing so would change the lighting in the room. In fact, it seems at times that all Catharina really cares about is having lots of children.

In contrast, Maria Thins makes the financial bargains for Vermeer's artwork; she understands that it takes considerable time for Vermeer to produce the caliber of work that he does, and she does whatever she can to make sure Griet is allowed to assist Vermeer in peace with his tasks. She offers quiet encouragement to the both of them (painter and assistant) and keeps out of Vermeer's way when he's focusing on his work.

Griet herself, like Maria Thins, has a natural eye for color, shadow, depth, and so on; she's also willing to learn from Vermeer, which adds to her natural understanding. Even before she becomes Vermeer's family's maid, Griet, having been raised by her tile-painting father, pays special attention to color, arranging chopped vegetables for a soup according to their shades and describing everything and everyone she sees in terms of color and shade. When she begins assisting her master, she pays attention to the relationship between the fineness with which she grinds the materials for the paint and the intensity of the color it produces, among other such details. Paintings themselves, whether finished or in progress, affect Griet deeply. She can barely tear her eyes away from the paintings on display in the Vermeer household, and the paintings of the Crucifixion disturb her. When in Vermeer's studio, she behaves with quiet reverence, demonstrating her profound appreciation for Vermeer's art, both process and product. Even the objects themselves that have been on display while Vermeer painted their likeness are handled with serious respect by Griet, whose appreciation of art is, overall, as deep as Catharina's is shallow.