I love the Nike slogan: "Just Do It". I find that taking that first step is usually all the motivation I need. It's forcing that first step that's the hardest. Also, remember "baby steps". Thinking about all that's left to do can be overwhelming. As Lakekids suggested, just take one step at a time. You'll get there. I also have 3 large quilts in various stages that I need to get to work on.
Jeanne ~ So sorry you lost your sister so young! I know that you will make a lovely quilt to continue the wonderful tradition she started.