I suspect it's just as well you haven't told her your feelings; in all likelihood, you would have made her heartache even worse. As the Bible wisely says, in human relationships there is both "a time to be silent and a time to speak" (Ecclesiastes 3:7).
I can only speculate as to why your sister feels the way she does; she might not even know herself. But we often forget how absolutely final death is, and how it closes the door to any further contact on earth with someone we loved. In spite of their problems, your sister may still have loved her husband, and now that he's gone she knows that any hope she may have had for a better marriage is shattered.
Grief also can bring with it a sense of intense loneliness, and I suspect this is the case with your sister. Although they had a contentious relationship, they were always together—and now she faces the future alone. Remember, we don't grieve for the one who died, but for ourselves and the loss we feel.
Ask God to help you be an encouragement to your sister. Remember Jesus' words: "In everything, do to others what you would have them do to you" (Matthew 7:12). Pray for her, be a friend to her—and most of all, point her to Jesus, who alone can fill her heart with hope instead of hurt.