I tried 'who is her son in law' and it was wrong (but maybe because I forgot the hyphens???) It would be good to have a "TIPS" link for this whole relationship issue because the hunt-and-peck, trial-and-error, guess-and-get-it-right-sometimes way of learning is not very efficient! (Or fun!! :( )
There seem to be a lot of same/different sex or same/different generation issues involved, but I don't have a clue when they apply. Very frustrating!
I suspect the issue is that it doesn't specify a gender, so it's not entirely correct to say either son-in-law or daughter-in-law. But at the same time, ʻo ia is neither male nor female and all of the sentences accept she or he interchangeably, so I'm not sure why they wouldn't accept daughter-/son- in-law unless they intend to make the distinction later. For that matter, kāna doesn't make any female/male distinction.
As far as I can tell, same/different sex distinctions apply for brothers/sisters only. If there's anything aspect you're having trouble with, I'd be happy to lend whatever help I can.
I agree - not clear why 'who is her daughter-in-law" is not correct? I was thinking maybe it has to be the same-sex or opposite-sex rule that Hawaiian seems to have, but unless there are some tips somewhere that I keep missing then this is totally shooting in the dark (with an occasional hit and a lot of misses!!)
I don't think it has anything to do with the gender of the speaker. Hūnōna doesn't appear to convey any information about gender (one would specify hūnōna kāne "son-in-law" or hūnōna wahine "daughter-in-law"). Presumably they want to emphasize that point, so they're not allowing the gender-specific terms.
In case it's useful, I found a reference with an in-depth discussion of Hawaiian kinship.