What is the infinitive of Esse?
Sē here is an accusative reflexive pronoun referring back to the subject of the main verb i.e. Iūlia ; esse is the infinitive "to be." Note that the tense of the infinitive, translated into English, is relative to the tense of the main verb.
What is Ecce Latin?
Ecce is the Latin word meaning behold. It occurs in the following phrases: Ecce homo, Behold the man, the words used by Pontius Pilate when he presents a scourged Jesus Christ to a hostile crowd (in the late-4th-century Vulgate Latin translation of the Bible).