My extended family has a handful of doctors in it, and every time anybody in the family has a medical problem, every doctor in the family is somehow notified and asked for an opinion. This is more true if it's something that's related to my specialty. It's also complicated by the fact that the other doctors in the family all work in other countries (mostly Pakistan), so they often ask me something like "how do they handle this problem in America?" That puts me at somewhat higher pressure to give advice, since the Pakistani medical system is focused more on treating things like malaria and tuberculosis and gunshot wounds, and not as much on things like sleep apnea, hypo/hyperthyroidism, etc. In those cases, I usually ask enough questions to ensure that my relative is being treated by somebody who is appropriately qualified, and then I'll usually give that doctor the benefit of the doubt whenever possible (i.e. the doctor providing inappropriate treatment is probably less likely than the possibility that my relative is making a mistake in relaying information to me).
If people aren't specifically asking for my advice, I usually won't go out of my way to get inovlved, unless I notice an overt problem and my relative isn't recognizing the significance of it. For instance, med noncompliance or procrastinating instead of making a doctor's appointment, both of which are often caused by underappreciation of the potential seriousness of the problem, etc. Like a relative with a long history of mild-mod asthma who now has a lot of risk factors for CHF, but is dismissing his SOB as just asthma even though he's not wheezing. Or somebody who isn't using CPAP because they think that sleep apnea is "just a sleep problem" and don't recognize the importance of pulmonary hypertension, etc.