Thank you for all the input.
At least now I know that I am not overlooking some possibility to add a new relation qualifier (except under "Comprises or Encompasses"), so I won't spend any more time looking for that. :-)
I will stick with
Family relationship to /
Other Family of for now. That works well enough.
It sounds like you, Rob, are talking about some sort of pedigree presented in a graphical interface - some time in the far future. If I understood that part right (probably reading too much into it), it sounds very interesting!
But first: Calendars! 