I never did finish the very first quilt I ever started and that dates back to '89. I also have many WIPs and UFOs. That said, I did complete close to 30 large quilts in my time, so I guess I can't complain.
All I can recommend is to find joy in the process rather than dwell on the negative. If you associate the making of this quilt only with the passing of your sister, you may never get it done because the project does not give off positive vibes. If you look at the quilt as a healing or an escape, you are more likely to finish. Sewing a quilt can be quite cathartic.
Just hang in there and when you get stuck - give the QB a shout for inspiration and motivation.