The issue is with the fact that all of these families of yours are considered "pre-existing." I've reached out for help about this too and, depending on the extent of your families the solution may be frustrating.
https://answers.ea.com/t5/General-Discussion-Feedback/Cannot-edit-pre-exisiting-sims-relationships/m-p/10647737 This is the thread I posted about it and you can read the solutions from Psychotops, however a summary is that you would have to download MCCC, delete all pre-existing relationships between every single sim (if your families are large this may not be the easiest so I recommend saving back-ups) and then you have to go back through and re-work all of the relationships, including those which connect the different generations together.
Unfortunately cas.fulleditmode cannot bypass pre-existing relationships, and it only works between sims who have no pre-existing relationships. E.g on my post I explained I was trying to give Lilith and Caleb Vatore some parents, however the ability to make both parents related to both Vatore siblings was blocked by the existing family relationship between Lilith and Caleb. I had to delete this, and it was fairly simple considering it was only 4 sims, however I understand you have multiple large families and so it may take some concentration and complete dedication of your focus to fix this correctly.
