- The only "work-around" I've found right now is to:
1.) Evict your family from the house
2.) Edit the family in household management
3.) Save your family and upload them (without changes)
4.) Download them back into the family (now you have 2 of each of them)
5.) Proceed to change relationship (which for some reason is now unlocked) and delete the new set of the family.
6.) Move them back into the house
The downside? Not the same amount of family funds (but hello motherlode/kaching/rosebud cheat & freerealestate cheat) :)
(Perhaps this also works with just adding 1 person to the family and removing him/her again after setting the relationship, I haven't tried, but the above worked for me ^^)