It's getting shockingly confusing,
From 1985.
Kim's mother had an affair with a Japanese man resulting in Kim (could this be the Udagawa connection)?
There was some confusion with her mother and stepfather during which Kim ran away from home, I believe Scott was blamed for the pregnancy but that got sorted out.
Kim was pregnant by Brad (Not Willis or Satchwell) Brad was trying to sell Kim's and another girl's babies. The original Scott helped her, including getting her a room at the motel then opened by the Udagawas. Kim was reconciled with her mother and they moved away, presumably the twins were the result. (is this the Key clue)?
Present day 32 years on.
The twins have a Japanese great grandmother, presumably from Kim's father's side.
Their mother and great grandmother refuse to tell them who their father is. The great grandmother told them his name was Brad but nothing else about him.
They start investigating and things seem to be pointing to Scott being the father.
Paul is now suddenly saying that he's the father. I don't believe that he is. He is either trying to protect Scott or going along with something Kim suggested because he doesn't like the idea of Amy being with Leo.
If they do carry on with the Daddy Paul story, they will have to explain the confusion pretty convincingly.