The date of Tarondor's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
Tarondor's name seems to contain Elvish elements for both 'King' and 'Land', and it may be that its intended meaning is simply 'King of the Land'. The resemblance of the combination '-ondor' to the name 'Gondor' is probably coincidental, especially as there was an earlier King, also named Tarondor, who ruled Arnor in the north.
In volume 12 of The History of Middle-earth, Tarondor is referred to as Minastan's eldest son. That implies that he had at least one younger brother, and possibly other siblings as well.