Although Susan doesn't see her family much after moving out west, her sister Bessie and brother-in-law John provide her with a consistent connection to home.
But, Bessie and John are a lot more complicated than that—they have their own hopes, dreams, and aspirations. For example, Susan learns during one of her visits that John "envies Oliver" for chasing his dreams out west while he sits "smothering" in Milton (4.7.39).
So, what's stopping them from packing up and leaving? Well, we can think of plenty of reasons: namely, that someone in the family needs to stay in Milton to take care of the family's holdings, especially now that Susan is M.I.A.
At a certain point, it actually seems like Bessie and John's dream is going to come true when they purchase stock in Oliver's company and plan to move out to Idaho. Sounds easy peasy, right? Wrong. Due to a minor flub by Oliver, Bessie and John's land is stolen by a greedy lawyer, and their dream suddenly becomes a nightmare.
We don't actually see how Bessie and John's story ends, though we can only assume that they move out west at a certain point. In the end, however, their tale shows us that even the minor players in Angle of Repose have fully fleshed-out experiences.