Yes, I can understand how you feel, and I know many people reading this understand also, because they too are grieving the loss of a loved one. Holidays often are very hard for people in your situation, and your letter should remind all of us to reach out to those who are facing heartache and sorrow.
But God understands your grief, and He wants to help you. Remember: God knows what it is to grieve the loss of a loved one, for He had to endure the death of His only Son—something we especially remember at Easter time. And when hard times come—as they do to all of us—God does not abandon us. You can depend on His promise: "When you pass through the waters, I will be with you; and when you pass through the rivers, they will not sweep over you" (Isaiah 43:2).
Let me encourage you to accept your nephew's invitation. I know it would be easier just to stay home, but sometimes the hardest thing to do is also the best thing to do. We need other people, and even a few hours with other members of your family can begin to ease the hurt.
Above all, focus this Easter season on Jesus Christ and His death and resurrection for you. He was God's gift to us, and because of Him we can have hope—hope for life right now, and hope for eternity. Is your faith and hope in Him?