So much depends on family dynamics. The default answer is "Just don't do it." You'll never make a right mess of things if you go that way. On the other hand, for some people given particulars of their families, it *can* work out, but you can't count on that in the general case.
With my family, if any of them decided they wanted to dive, I'd teach them without a second thought. All our various personalities and history fit together in such a way that it would work (and already has with two of them). On the other hand, I know more than a few friends who I would pass off immediately without ever getting near a class.
So, the simple answer is don't try it, as it's likely to end in tears. The nuanced answer is *sometimes* for certain combinations of personalities, it can be *wonderful*, but if you guess wrong, it's likely to end in tears. And my personal aside is that if another instructor came up to me and asked me to teach their family member, I would gladly do it free-to-the-instructor (they pay expenses, of course) as a favor to them, and if they want to drop in to help me in the pool sometime, that would be a nice but not at all expected thank you.
(There are two kinds of free, by the way. There's free, meaning "I'll do the bare minimum required to get the card, since I don't value what I'm not paying for." And there's free, meaning "I'm doing this at no charge because I believe it's important enough for me to volunteer my time." If I do it free, it's the latter. On the other hand, if the student is of the former definition, they can pay full-rate and I'll buy the instructor lunch on the next dive trip.
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