I think if you have to ask this question, you should avoid using it. The reason being: what if the letter were to somehow come up that it is your uncle and you were mistakenly seen as being dishonest, or nervously told a white lie out desperation to cover the fact that he was your uncle? Would it be worth risking dental school? Although it is unlikely and maybe not a concern, I think not disclosing information intentionally could be viewed as simple dishonesty. I would call anonymously and ask if it's ok to have your non-blood uncle write you a letter because you've had the most experience with him.