Would you be shadowing them, or just getting a letter? If you shadow them, I would for sure get a letter, otherwise it just seems pointless.
My uncle is a urologist and I plan on shadowing him and getting a letter too. Of anybody I could shadow, he is probably the most qualified to write about my intelligence and interest in medicine.
The entire point of shadowing is to spend time around a doctor. Whether it is a family member or not should not matter. And conflict of interest? If I was going to shadow my uncle, but failed to show up, and acted bored and pissed off during the shadow....he wouldn't give me a letter. He'd tell me to get my act together and figure out if I really want to practice medicine!