Owning multiple homes

I found this post by searching for how to buy a 2nd home in Sims 4, as they did in Sims 3. I am out of room in my town, as the family's kids all got married and moved into homes and had their own families. Well, I have twin grown boys who met twin ladies (I created the ladies), and in the same home and with kids now. The one twin, Charles, met a gf, and they have 1 available lot left, as their love nest, but would like her to move into it, but how? I can't get her to move in first, the wife will get very upset, and the family also already has 8 ppl in it.
Also, there must be an option to have more than 8 people in a family?