There were no older siblings, the birth record for Holm says he was the first child born to the parents.
There are no siblings listed in the 1910 census, when the family was living in Oslo. There could be siblings born after 1910, but since they were living in Oslo, this would mean a dozen possible parishes with hundreds of pages of records to search.
Accordin to the 1910 census, they were living on the street "Grønlandsleret", which is in Grønland parish.
The 1913 Oslo city directory shows that Fredrik is still living there. It does not say if Fredrik and Thora ever got married or were still together in 1913. But is they did marry or have more children, the records should be in Grønland parish, Oslo.
He was no longer at that address in 1916.