The question is, will your two cents do any good? Or is offering well-meaning advice about bad habits at a time like this as futile, and emotionally explosive, as preaching politics and religion?
"When you confront someone strongly, you raise their defense mechanisms. They shut down and don't hear you," said Harris Straytner, an associate professor at the Mount Sinai School of Medicine in New York and a regional vice president of the Caron Treatment Center. "If someone is an alcoholic, for example, you don't say, 'You drink too much,' at a holiday gathering."
Instead, said Straytner, you can use these get-togethers as an opportunity to express interest in how the person is doing, and make a plan to get together another time.
Using a technique he calls "carefrontation," Straytner suggests having a discussion away from the crowd and in a less intense setting. "You can say, 'I love you, this is just between us, I'm not trying to upset you, but I noticed that you're drinking more than usual. Are you OK?' " It is important to keep in mind that no matter how dim-witted they may seem, you are not telling your relatives anything they do not already know.
Furthermore, he said, no matter how persuasive you may be, your input will not magically make anyone decide to lose weight, quit smoking, or check into rehab. But finding the right words and tone to convey love and concern may help move someone a little closer to making changes for the better.
"There is no specific research to say when somebody should be approached," said Ann Malarcher, a senior scientist in the office on smoking and health at the Centers for Disease Control and Prevention. "You have to look for natural opportunities. . . . When the person is ready to make the decision to quit, you can offer to help."
Studies show that more than 60 percent of smokers would like to quit, more than half try in any given year, and one-third say that having the support of friends and family is extremely important, said Malarcher.
At this time of year, people are poised for change, said Paul Baard, who teaches motivation and management at Fordham University. "What you can do is give them a vision."
To effectively offer help, Baard explains, it helps to understand the three kinds of human motivation.
The first is intrinsic, or self-motivation, which drives you to do something - playing baseball, reading, volunteering at a soup kitchen - simply because you like it.
The second, he said, is extrinsic, doing something because it pleases others - such as baking cookies for your family or working overtime to impress your boss.
The third is "amotivation," or the absence of any purposeful drive. Baard gave the example of someone who decides to go to college because no one from the neighborhood has ever done so before.
It is rare that people are driven exclusively by one kind of motivation, he said, so adding some extrinsic influence can help fire up the engine.
"When someone starts a diet, it is often because their doctor scolded them or their wife shamed them into it. But they also may want to lose weight to be able to keep up with their kids or because it gives them more options in the clothes that they wear."
Baard offers another codified trilogy to help a would-be helper.
"Everyone has three innate needs," he said. "Autonomy, competence, and relatedness." All three, he said, "are as powerful as any physiological need." Any attempt to nag, cajole, threaten, or bribe a family member will feel controlling and imply that the person is not capable of handling the problem.
The key is to tap into the need to relate to others.
With New Year's resolutions only a month away, he said, this is a perfect time to form a self-improvement team. Beginning the conversation with your own objectives - say, trying to be less judgmental - can make it easier for the other person to make a plan, too.
"A vision of how things can be better is what changes a person," he said. "And you never know what will help make that happen. It can be as simple as a motivating word." In theory, anyway.
Some people are just too sensitive and the merest hint that anyone thinks they need to change in any way puts a match to the fuse. Next thing you know, the room is exploding in denial, rationalization, and finger-pointing fury.
Consider obese, alcoholic, crack-smoking Toronto Mayor Rob Ford.
Given his on-camera performances, it is hard to imagine him reacting well to his wife's saying she has issues, too, lovingly asking him if he has anything he would like to work on, and suggesting they embark on the journey together.
Sometimes, said Straytner, even the best attempts to be gentle and avoid triggering defensiveness will fail.
"Your loved ones may get mad," he said. "But don't be afraid. They'll still hear you and eventually will do something about it."