Rather than see Sariah as one whose faith wavered, we should rather see her in the light of a loving mother with tremendous concern for her sons. It is only after she fears that her sons are dead that she decries Lehi as a visionary man.
There is no indication that Sariah was hesitant to follow Lehi into the desert. Nephi tells of Laman and Lemuel's dissidence, but not of Sariah's - not until these verses. Sariah appears to have believed her "visionary" husband up until the time that her sons did not return when she supposed they should have. Sariah has the imagination of a concerned mother, and every day they were late multiplied the calamities which might have befallen them. In her self-constructed despair, she is certain that "my sons are no more". It is only at this point, when she feels she has lost her sons, that she begins to revile her husband, and to expand her concerns to the entire episode. After all, if her sons had died on an errand from Lehi, could he really be a prophet?