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? Below is a part of the guidelines from a grant:

"Research assistants (RA) shall not be appointed from amongst members of immediate or extended family of the researcher."

I know it's to prevent professors/researchers from misusing it for personal benefit.

But what if the two researchers (principal investigator and potential RA) are in the same field, doing and discussing the same project; and the potential RA is among the best in the field; and no other people/students here are capable of doing that project? 

I've seen similar question in another thread, but it's quite different.

Here, we are talking about deep theoretical research; with lots of advanced mathematics and application to a field that is really rare even among researchers in the world. NOT about technical jobs or plain research tasks that anybody can do as long as we train them. 

And the topic is quite hard, require extensive mastery - and already mastered by the potential RA through his extensive years of research. Note that the RA here is not a student, but rather has a history of working as a research scientist in a great research institution. The topic of the project itself was proposed based on the discussion between the PI and the potential RA.

Of course anybody can train but takes lots of time and effort, but the students here lack even the basic knowledge of the topic. So those students needed to study from scratch for months or even years. So it wastes time and effort to hire and guide student who don't know anything, and PI need to chase the project's deadlines too. The project will succeed more when working with the researcher who already mastered the topic.

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?

(Isn't this kind of guidelines is an injustice towards a very good potential RA, but s/he is dismissed just because s/he's a family member of the principal investigator? Or is it possible to be amended if being discussed case-by-case with the higher-ups?) 

There're also spouses, or fathers and sons/daughters who are doing the same researches together. It's sad if they're being discriminated by this 'good' rules. Moreover, there're lack of research jobs for postdocs/phd graduates, hence come the two-body problem in academia.

Of course it should be discussed directly with the university/institute, but any enlightenment is very appreciated.
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.