I'm so sorry for the grief you're going through.

I've been through cancer diagnosis and deaths many times in my family. Let me be honest with you. Sometimes people die quickly. Other times they do not. Many times people with serious cancers live for years. Treatment is hard, but it really does prolong life. Your family is correct by telling you to be more positive. Yes, your uncle will die eventually, but we all will. In the meantime your uncle is alive and deserves to have his aliveness honored.