This is a question of mine coming from my time playing Rome Total War. It was always weird to me that each faction of the Roman empire could expand at their own will but ultimately answered to the SPQR (senate)
Any further information on this would be awesome.
Rome: Total War is a game, and it deviates from actual Roman history in some major ways for the sake of gameplay.
The 'families' you're talking about are properly called gens. A gens is a group of people who all claim descendancy from the same person. Obviously that makes them family, but not necessarily very close family as the common ancestor might be many generations removed. Many gens consisted of several distinct branches.
The gens played an important role in Rome's society, but they mostly involved social and religous aspects. Members of a gens were expected to take care of each other, and to listen to the elders of the gens. I'm not very knowledgeable on the social aspects of Roman society though.
What I can say is that on the political and military side of things, gens did not play a huge role. Every year individuals were elected to magistracies. Perhaps these individuals secured their election with the support of their gens and their clients, but it was always the individual who took up the magistracy. And the next year (usually) some other individual would take up that magistracy. Never had a gens a monopoly on certain magistracies or certain areas of the empire, so the idea that they would expand the empire "at their own will" as a gens is nonsensical.
Interestingly, the elected invididuals did sometimes expand the empire at their own will. But these were always the actions of the individuals, not of a gens.
/u/Polybios got it (as did /u/DBHT14--much as I enjoy RTW it's laughable compared to reality). But I'd like to add that while there is a gens Julia (Julii, according to CA, though you don't often talk about a gens in the masculine plural) there is no gens Brutia or gens Scipia. CA misunderstood the Bruti branch of the gens Junia, the plebeian family that was one off the oldest families of the nobiles a family that included two of Caesar's murderers, Marcus and Decimus Brutus, for a region of southern Italy, Bruttium. They have nothing to do with each other, CA just doesn't know Latin. The Italic tribe that lived in Bruttium were called the Bruttii, but they have no connection with the Brutus branch of the Junii--the gens Junia was the oldest plebeian family in thr nobiles, so old that they claimed (erroneously) that they were originally patricians who had been demoted. They were in Rome well before there was any contact whatsoever with Bruttium. To reinforce the fact that CA doesn't know Latin, consider that the "Scipii" are supposed to be the family of Scipio Africanus, among others. Scipio is a third-declension noun whose nominative plural form is Scipiones and whose root is Scipion-. "Scipii" would be the nominative plural masculine of "Scipius," which is not a word or a name--CA apparently thought that all Latin names ended with "-ii" in the nominative plural. Amusingly, CA didn't seem to understand naming conventions either, since the gens Julia is a full gens, but the Brutus branch and the Scipio branch were both branches of the gens Junia and gens Cornelia respectively--they aren't really parallel
Even the names are wrong. Brutus was one of the Junii; Scipio, of the Cornelii. If you want to get closer, it was Scipiones, not Scipii.
I'm glad the game was made. I had a lot of fun playing it, and especially it got a lot of my friends interested in Roman history so that they came to me to ask questions. I love answering questions! Unfortunately in the case of Rome: Total War almost everything you learn is wrong - except maybe geography.
These three families were not different factions as Total War portrays them. They were related and had similar interests and competed for honors and so on and so forth. The Scipiones were also most powerful a generation before Caesar and Brutus entered politics.
Caesar did indeed seem to conduct his own foreign policy while he was governor of Gaul (invading Britain and Germany for instance) and his actions were discussed in the Senate, but no action was taken in the end, so we must assume a majority didn't think he did anything wrong.
Governors would often have to answer to the Senate - in particular I'm thinking of Cicero during his tenure in Cilicia, in which he found the loans Romans gave the locals to be at an incredibly high interest rate. We have his letters to document he tried to reduce it but found himself limited by the decrees of the Senate on that matter. The lender, btw, was Brutus.