Did you talk to and visit Nia and Rebecca every time you had the chance?
Maybe the final trigger is in the dialogue before the wedding. I asked Rebecca for example, if she doesn't find it disturbing, marrying my family and having so many other wifus. And she just said they don't have a problem with that here.
As said before, I was all the way on hyper-polygamy and asked everyone if they're fine with this.