Jennet is pretty much just there to provide foreshadowing of terrible things to come. In Chapter 2, she tells Davie that "Blood built [the house of Shaws]; blood stopped the building of it; blood shall bring it down" (2.18). She's a wrathful tenant of Ebenezer's, which says something about his character. But there's also some nice indication of what's going to happen later in the story: "blood stopped the building of [the house]." As in, bad blood between Alexander (Davie's father) and Ebenezer broke the family in two and turned uncle against nephew.