I am sorry your Mother-in-law passed before you completed the quilt you were making for her, however, it sounds like it gave her great comfort to know you were making it. You should complete the quilt in her honor. Although the family knew you were making it for her, it would not be wrong for you to keep the quilt for yourself or a member of your immediate family. Do you have children that would like to have it as a memory of their grandmother? Is your father-in-law still living and were they together when she passed? Having it might provide comfort to him. Or, like others have said, you could donate it to someone else who has cancer and needs to know someone cares.
When I have something that I need to work on but want to keep putting it off, I put it at the top of my list. I set aside a few minutes to work on it 1st thing every day. After the alloted time, I stop and work on something I enjoy doing to reward myself. Before long, the dreaded project will be completed and you can move forward on another project you will enjoy.