This surname appears to originate from one "Justinijani iz Venezia" aka "Justinian from Venice", who is mentioned in Andre Jutronić's book from 1950 about the settlements and inhabitants of the island of Brač ("Naselja i porijeklo stanovništva na otoku Braču".
Although Jutronić notes that the name (which was the basis for the surname) was recorded as early as 1622, he does not give any information regarding the source of his information, so we do not know whether it came from the birth, marriage or death register of Bol or if it was in the Status Animarum.
Immediately after the 1622 mention of "Justiniani iz Venecije", Jutronić provides a list of the Bol contingent of the crew of the Venetian war galley of Brač in 1625. There is no mention of any "Justiniani" in the list for Bol. As the list is based on the "bračkih pučana" (male commoners of Brač), who were those inhabitants of Bol being obliged to serve as crew (rowers and matelots), one would be excused if one assumed that the said "Justiniani iz Venecije" was somehow exempted from this duty.
However, in another document by Jutronić, which gives considerably more detail about the crew of the war galley of Brač and derived from a manuscript by Juraj Mladinić-Mladineo of Pučišća dating back to 1643, this "Justiniani iz Venecije" (aka "Giustinian da Venetia") appears in the list of the crewmembers of Nerežišća, ("Novačenje u Nerežišćima" aka "Galeotti di Neresi") which was at the time, the principal settlement on Brač. When referring back to Jutronić's book on the settlements of Brač, in the summary listing of crew members from Nerežišća, there is a "Justinica iz Venecije", a subtle change of name, but undeniably referring to the same person. The manuscript by Juraj Mladinić-Mladineo may be found in the Museum of Archeological Monuments in Split,
So now we have a "Justiniani iz Venecije" who appears to have been resident in Nerežišća, and who probably got married in Bol. As he was obliged to serve as a rower for the galley of Brač,, he was clearly a commoner and probably his origin was from Brač. Although he is noted as "being from Venice", we have no confirmation that he was actually born there. Even if this was the case, because there was a sizable Dalmatian presence in Venice, it is probable that his parents were from Brač.