I think the uncle lost his family (including his sister, ripped photo? girl in photo look "kinda" similar) and tries to revive her. Summon doggo girl instead, doggo girl probably tell him she can't revive the dead, and he assume that there other that can and continue tries summoning stuff later on. Doggo girl fall in love with the uncle so she tries to be friendly by turn into the sister. which cause a fallout with the uncle instead. Uncle still try to revive his family and this is where Yuu come in.
either that or he the same as Yuu. But the contract was fulfill and she have to go back. When Yuu summon Chiyo, she slip through too.
edit: also all the bad end foreshadowing, the contract said he was to give up something he treasure the most, in which case, Chiyo was the most likely target now.