There is a point, unfortunately, to merely walk away from a sibling and consider him or her to be a non-entity. In my case, my nearby sister, despite entreaties from me to help in the caregiving role, and well as appeals from my ailing mother to help me out, my sister outright refuses.
Yes, we can spend precious time and emotion in maintaining some sort of relationship through updates and such, and other tips offered here. But I think that at some point, one has to realize that time and emotional capital is scarce, and that is it useless to spend it on a sibling who, at the end of the day, has decided to divorce himself or herself practically and emotionally.
I've made the decision to divorce myself from her, and quite honestly don't expect to speak with her for the rest of my life. There's no point.