I totally agree Voldemort does not understand motherly love or any love for that case and therefore again he underestimated it by sending Narcissa to check if Harry Potter was dead, but she was only thinking of how quickest to get to Draco. But i do not believe it was the reason that Voldemort did not give Narcissa the dark mark. Narcissa may be very proud of her bloodline and she may believe in those ideas but i don't think that she has the oomph to kill someone, not unless it is for Lucius, Draco or maybe Bella. So I'm not sure that it's to do with Voldemort's trust in her but maybe he just didn't see her useful unlike her sister, to him Narcissa may have just looked like a house wife and mother.
From what we've seen of the Death Eater's, only two known women -- Bellatrix Lestrange and Alecto Carrow -- have the Dark Mark. My personal opinion is that Voldemort did not give the Dark Mark to Narcissa because she is a mother. Well, the only two women with the Dark Mark are women who, to the best of our canon knowledge, have no children. I think Voldemort may have not given the Mark to mothers. Think about it. Voldemort is not known for being exceptionally fair, and Voldemort as a huge sexist doesn't seem very out of character to me. Voldemort doesn't understand a mother's love (as demonstrated about a hundred thousand times in the books), and he would probably not value mothers enough to put them in the inner circle of Death Eaters who took the Mark. I, however, think Narcissa was heavily involved, maybe enough to deserve the Mark, but Voldemort never valued her enough.