It's an assumption but it's VERY heavily implied by chapter 21.5, which shows her at the beach with an unknown man when she was Touko's age, and the second half of chapter 22, when she and Touko are talking about Touko's father.
Basically Hiroto is NOT the "Papa" being referred to in the title.
Everything that's happened so far pretty much lines up perfectly with the idea that the mother is looking to either recreate the environment in which she was groomed for her own daughter, because she's romanticized it for both of them, or that she's somehow convinced they can more literally manifest her dead father into a new body if there's a supernatural twist. That's what the manga's title is a reference to in my opinion: That they're trying to bring him back, either literally or metaphorically.