I would think that her paternity would be important regarding the legal disposition of his property in Boulevard.
Unless he left a will his property will go to his sister Lora — unless Hannah turns out to be his daughter.
Why would she and the Anderson family not want to know???
[actually I do know but I am just saying that to be facetious]
It's entirely possible that they feel absolutely sure in the paternity already. Perhaps Hannah strongly resembles other family on the paternal side of the family.
It's possible that Uncle Jim developed an obsessive fantasy that Hannah was his daughter that had no basis in reality.
My middle child and my sister's middle child look more like siblings than they do with their actual siblings and strongly resemble a great grandmother. My youngest is the spitting image of my mother and it was obvious on the sonogram of her facial bone structure in utero.