It took me quite a while to get over the deaths of my mum and dad but it does get easier as with any death. I also felt that my parents were the one holding my relationship together with my brother and sister. My brother is a very successful executive in the airline industry and my sister is a very quiet English girl. My brother travels regularly from USA to Asia so I do see him a couple of times a year but I do feel my relationship with my sister has become strained since my parents death and especially since I had the twins - I don't really know what her problem is.
I think it is important to make an effort to keep in touch with your sister and to carry on that side of your life.
Again, I'm so sorry for your loss.