(MONEY Magazine) - Q My brother, a successful dentist who fancies himself a hotshot investor, recently bragged that he's about to buy a large parcel of undeveloped land. I'm a C.P.A. who knows a bit about investing in raw land, and I also know that my brother has no idea of the risks involved. Am I obligated to tell him, even though I'm sure the last thing in the world he wants to hear is his big sister warning him, yet again, that he's being foolish?

ANSWER Why is it such a thankless task, saving the people we love from their own worst impulses? Fortunately, we are rarely obligated to do so. Your brother is an adult, and adults are entitled to make their own decisions and their own mistakes. In fact, once they both grow up, big sisters need to let their little brothers do just that. Only two circumstances obligate you to give your brother your opinion: if he asks for it or if you're sure he's about to seriously jeopardize his family's financial resources.

Obviously, you're off the hook on the first count. And since we assume your successful brother can afford to make a mistake, you're probably off the hook on the second count as well. So unless you have good reason to believe a calamity could occur--if, say, your brother is betting the kids' college funds or your sister-in-law has confided they're over their heads in debt--you can in good conscience keep your advice to yourself.

And who knows? Maybe if you withhold your opinion, your brother will find himself missing your good counsel instead of resenting it.