As Rome, can I prevent civil war?

As ShinoGH said, "The only way to avoid civil war is to maintain a balance between all houses involved in a faction. Not too strong, yet not too weak either. "

Refer to this link for this and for more information concerning your question:

http://forums.totalwar.com/showthread.php/86419-how-to-avoid-roman-civil-war

OR

In the TW series, there are editors you can use to change scripts OR scripts that can be changed with the help of notepad++. Try this link for the script that can be changed (I haven't found an editor, as I haven't actively looked. If you can't find one, you can try notepad++ as it can work correctly in SOME situations). ALWAYS MAKE a backup of the file you want to edit!!!:

http://www.twcenter.net/forums/showthread.php?628192-Altering-what-units-spawn-in-civil-war