I'm sure your uncle is very thankful that your aunt had committed her life to Christ, and that she is now safely in heaven forever. The Bible says that because of what Jesus Christ did for us we have "an inheritance that can never perish, spoil or fade—kept in heaven for you" (1 Peter 1:4).
But the death of someone we love is still painful to us, even as Christians—not because we fear for them, but because of the empty place they leave behind in our hearts. The Bible says we don't "grieve like the rest of men, who have no hope"—but we will still grieve (1 Thessalonians 4:13).
The more we loved someone, the more we will miss them—and the more we will look forward to seeing them again in heaven. When Jesus came to the tomb of his friend Lazarus, we read these words: "Jesus wept" (John 11:35). That is the shortest verse in the Bible—but it contains a world of truth. If Jesus wept over His friend (who shortly would be raised from the dead), won't we also be touched by the death of someone we love?
Pray for your uncle, and do what you can to be his friend. In addition, urge him to reach out to others who need help and encouragement, including those who are going through the same experience he has (his church may offer opportunities for this). Not only will he help others, but he will help himself.