Sadly, it's almost impossible to help someone who rejects every attempt we make to help them; often it's even harder if it's someone in our own family.
Why do some people stubbornly reject help, even when they clearly need it? It may be because of pride, or a refusal to face the consequences of what they're doing, or laziness, or any of a number of other reasons, but the result is still the same. The Bible compares such a person to "a cobra that has stopped its ears, that will not heed the tune of the charmer" (Psalm 58:4-5).
Does this mean you can't do anything to help your uncle? No, not at all. First of all, you can pray for him; you may not be able to break through the barriers he's erected, but God can. Pray not only that he'll face his problem and seek the help he needs, but that he'll turn to God and give his life to Christ. Pray too for yourself, that you and your family will have wisdom in knowing how to deal with him.
Then talk with other members of your family about your concerns, and seek their help. Alcoholics often refuse to admit they have a problem, but if others join with you to confront him and demand he get help, he may listen. Let him know you love him; if you didn't, you wouldn't care what happens to him. As a footnote, your pastor or doctor may know of successful treatment programs in your area.