Is it inappropriate for a PI to hire the most suitable available researcher as an RA, just because s/he is his/her family member?
Is it inappropriate for a PI to hire the most suitable available researcher as an RA, just because s/he is his/her family members?


Finding the most suitable candidate is a very hard problem, so lets be a bit more precise: 


  Is it inappropriate for a PI to hire the researcher (s)he feels is most suitable as an RA, just because s/he is his/her family members?


Now the problem becomes clear: Can the PI be unbiased when determining which one is the most suitable candidate if one of them is a family member? My suspicion is that for most people the answer is no. 

There are lots of other, more practical, reasons why it is a really really really bad idea to hire a family member, but those are discussed in the question you linked to. 

One of these that you should consider is to protect the reputation of the RA/family member who would be hired. Scientific communities are like villages: people gossip a lot. If you are hired by a family member there will always be the suspicion that you were hired because you were a family member, i.e. you weren't good enough to make it on your own. Once you got that stigma, it is very hard to get rid of it.