I have always wondered about this. I know it is in the T&C's, where there is even a detailed family tree diagram showing who is elig. But in reality, do they/can they really check? Let's take an example: per the chart, I can redeem a ticket for my brother-in-law (my wife's sister's husband). Now, the only way to 'prove' the relationship would be with ALL of the following docs:
1) my marriage cert to prove I am related to my wife
2) both my wife's and her sister's birth certificates to prove they are related
3) my sister-in-law's marriage cert to prove my brother-in-law is really who we say he is.

Now, let's be real!!! Who is going to bring all of these to the airport with them!!! My guess is they put this in the T&C's to discourage people from doing it, but in reality they never ever check. Anyone have an experience where they were checked? In addition, if you used your points with ANA to redeem for a partner award, no way the partner is going to check or care!!!