What if I don't feel that my "closest relative with progeny" is in any way related to me?

I don't care about inclusive fitness. Let the nature do that trick. I care about reproducing something resembling of myself. If my nearest genetic relatives don't resemble myself in any way, I don't care if they reproduce.

Certainly. But then I'd say that a huge portion of what makes you you is not genetic at all, and like I said much earlier you can always pass on memes.

I can give memes to other unrelated people but I don't know whether they use those for good.

Children are safe bet in that regards (if you failed - means you just passed your failureness to them, good)

I don't see why that doesn't mean you should work harder at it. Maybe the marginal benefit is minimal, but if the marginal benefit of everything else is zero then it might still be the best option.