The Myth of Unity
It is our common practice to talk about noble houses (and to a lesser extent, guilds and the Church) like they were modern nations - states that possess territory, have a population, have a military, and so forth and so on. This linguistic shorthand, while convenient, is not exactly accurate.
In the 21st century, the United States of America - or any other country - is importantly unified in a fashion that noble houses are not. If I happened to be in possession of an army, and I attacked (for instance) the city of Fresno, California, this is not a conflict between me and Alan Autry, the mayor of Fresno - this is a conflict between me and the United States, and I can reasonably expect the Marines knocking on my door quite soon.
Not so in Fading Suns. If, for instance, Baron Alistair Farfenugen rules the city of Frieze, and I - Baron John Montegue - attack Frieze, it is not House Montegue attacking House Farfenugen but rather I, Baron John, attacking Baron Alistair. As a nobleman, the troops I raise from my fief are my own, and the level to which I am accountable to my superiors for their actions is weak at best. Indeed, it is not at all uncommon for nobles of the same House - even owing fealty to the same overlord - to go to war against each other over territory, prestige, or points of honor, something that would be unimaginable in the modern world. More importantly, in doing so the two peers are not breaking any particular social rules. Certainly, their overlord is likely to be upset that his vassals are fighting, and is definitely going to send what envoys he can to put a stop to it, but (in most Houses) he has no more right to order them to stop fighting than he does to order two nobles of an entirely different house to cease their conflict.
It is crucial not to think nobles as members of a larger group - their House - but as independent nations in their own right, bound to their kin by custom, obligation and familial bond but not by law or hierarchy. When Alistair Farfenugen acts, it is Alistair Farfenugen acting, not House Farfenugen - instead, we can only really think of the House as a whole acting when it is the Prince himself from whom an action originates.
The bottom line, then: nobles are individuals first, and members of their House second, and the actions they take are - by common understanding - assigned to them rather than to their house. This sort of autonomy is both a blessing and a curse to the institution of the nobility, as it provides them both an excuse for the actions of their wayward peers and also frustrates them without a direct hierarchy with which to restrain those same lost siblings.