All this tells us is that unhealthy behaviors might have different effects in combination than they do alone. But while that might modify the degree of unhealthiness in specific cases, it doesn't change that these behaviors are unhealthy overall. And even if it did, that would only weaken the argument by suggesting that an unhealthy lifestyle might not actually be so bad.

Like (A), all this is doing is adding nuance to the idea of an unhealthy lifestyle. We already know that a good number of long-lived people have led definitively unhealthy lives, so we're not concerned with debating that—and this doesn't provide any further support for the idea that genetics, rather than lifestyle, might be the reason for long lifespans.

This is basically a mirror image of (B), just swapping out some unhealthy lifestyle factors for others. And exactly like (B), it makes no difference to the argument, because the author is trying to explain how people with certifiably unhealthy lifestyles still live so long. The exact combination of factors for specific individuals isn't relevant.

This is almost certainly true. However, the author makes no causal claim about leading an unhealthy life and living for a long time. We're concerned with the cases where people who live unhealthy lives do live to 100, and specifically with establishing that genetics are the responsible factor. This doesn't help with that.

This strengthens by providing additional supportive evidence. If a certain trait is genetic, then we would expect it to be shared between people who share genetics—like siblings. And if an overwhelming majority of long-lived people have long-lived siblings, it's probably genetic similarities, rather than any lifestyle differences, that matter.