The third one I adopted still displays as "N/A" so that's what led me into believing it might be a "2 adult limit". Now the house has the couple, one "N/A" adult, a child and baby (though, I could not continue the family line, as I could not manage to get a baby with Trishina, but I do not mind anymore)

Fun fact: the family tree displays the new children by the unrelated parents, but the couple is still Trishina and Tico (both deceased now).