Romans had very few first names, so they started numbering their kids.
Hence Tertius, Quartus, Quintus, Sextus, Septimus, Octavus, and their various diminutives.
So you might have brothers that are numbers. In other words, if Gaius Iulius Caesar had had legitimate male offspring, the first would have also been Gaius Iulius Caesar, the second Secundus Iulius Caesar, the Third Octavus and so on.
This gets confusing when someone like like the fictional Quintus Iulius Caesar has children. His first born son will usually be Quintus, but will his second be Secundus? No, he may be Sextus. However, if there is already a living Sextus Iulius Caesar, you may go with a diminutive, such as Quintillus, or you may cherry pick a later number out of order, such as Decimus.
None of this applies to women. Their father’s last name became their first name. Thus, Gaius Julius Caesar had a daughter, Julia. (What looks like a Roman middle name is actually the last name to our mind. Nobody called Caesar Caesar. He was known to his friends as Gaius and the public Iulius. The Caesar was appended from time to time to distinguish him from other people with last name. ) Ocatavianus had Octavia. Agrippa had Agrippina (the elder). She was a complete and total nut job - crazy as loon. Second and third daughters often had exactly the same name as their sisters, with a nickname that honors an ancestor appended. The second they were married, their first name was their father’s last name and their last name was their husband’s.
In this way most Roman names live on today as women’s names. Julii family, aside from having one son named Gaius Iulius Caesar, had many, many daughters named Julia. Thus Claudia, Amelia, Octavia, Melissa, and their associated names like Julie and Emily are with us today, particularly among folks whose native tongues are Romance Languages.
For the boys, we get Trey and Quin. A fifth. A cokehead named after a bottle. Go figure.