I actually did this. My uncle is an oncologist at one of the schools I am aplying too. I shadowed him on rounds, and later he wrote an LOR for me. He flat out told me that he was going to mention that he was my uncle, but said that he would write me a character reference. I already had letters from 2 faculty, 1 non-faculty, and 1 from my volunteer coordinator. All of this would be going along with my committee letter. Because I did not really need the 5th LOR, I feel that my uncle's LOR would not lower my stats and would serve as a good character reference. I do not know what your situation is, but as long as you fill out the requirements from the schools and have the one from your uncle as an extra, it wouldn't lower you much if at all.