It seems very odd that the father of Harriet and Alfhilde would be born in Florida, considering they were born in Norway. They both gave that information in census data.
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There is a slight possibility (some of this is close to accurate for these sisters) that the following family is Oliver's family:

FamilySearchô International Genealogical Index v5.0 Norway
Family Group Record
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Husband

Olaus Nielsen Melsomvig Pedigree

Birth:
Christening:
Marriage:
Death:
Burial:

Wife

Henriette Marie Halvorsdatter Pedigree

Birth:
Christening:
Marriage:
Death:
Burial:

Children
1. Ann Eliza Nielsen Pedigree
Female

Birth:
Christening: 14 APR 1867 Oslo, Akershus, Norway
Death:
Burial:

2. Oliver Nielsen Pedigree
Male

Birth: 1869 Fredrikstad, Ostfold, Norway
Christening:
Death:
Burial:

3. Alfhild Nielsen Pedigree
Female

Birth: 1871 Fredrikstad, Ostfold, Norway
Christening:
Death:
Burial:

4. Olga Nielsen Pedigree
Female

Birth: 1872 Fredrikstad, Ostfold, Norway
Christening:
Death:
Burial:

5. Karnil Nielsen Pedigree
Female

Birth: 1874 Oslo, Akershus, Norway
Christening:
Death:
Burial:

6. Harriet Mathilde Elisabeth Nielsen Pedigree
Female

Birth:
Christening: 22 APR 1875 Oslo, Akershus, Norway
Death:
Burial:

1. Alfhild Gunhilda NELSON b: 13 MAY 1871 in Christannia, Norway
2. Harriet NELSON b: 28 DEC 1875 in Christannia, Norway