Well she's trying to give Yui a father figure so that kinda method would be easiest versus some long con game of framing the wife up to be terrible (tho it's not like she didn't openly look down on Saeko for being a single mom/moving into a 'poor' apartment down the street lol)
lol the dad deciding to eat dinner at their house is even more convenient than the son so easily ceasing to think of his parents as family haha so silly