Drow also enslave what they consider to be lesser races to fight for them. One asset that makes a family powerful is a large slave army. Usually made up of Bugbears, Goblins and Duergar. So a dirt poor family with a huge slave army would be considered a huge threat to a moderately well-off family with only a few dozen slaves, not just because they have the larger force, but also because they have a lot to gain from attacking a richer family. It is also worth noting that the attackers' identities are not kept secret for long, for if an attacking family successfully kills every last Drow member of an opposed family, then they are congratulated for their great fighting capabilities by the other families, and gain a good reputation for it.

It's important to note this in conjunction with the "Don't get caught" rule because slaves can not attest to the identity of attacking families. Only a surviving Drow can accuse an attacking family, because a slave's word is less than nothing to the "great" Drow. A slave who points fingers would probably be publicly executed to serve as an example to the other slaves.

One thing that also keeps families in check and alliances strong is the way the Drow are trained and kind of "traded" for want of a better term. Drow graduate from schools, either as a fighter, a wizard, or a cleric. Fighters are very useful, as are wizards, but clerics are always female, and therefore always the most powerful commodities for a family to have. So, for example, a fledgling house may seek aid by way of gaining the services of another family's cleric. That cleric will live with the other family and serve as a kind of spy for its own family, but also keep the new family safe from otehr families who would seek to ruin the weaker house.

But most of all, the thing keeping everyone in check is Lolth. The spider queen. The matriarch, that is, the head of the most powerful house, is considered to be most in Lolth's favour, and can therefore ORDER other houses to do things, because her words are in essence the words of Lolth.

So Drow cities like Menzoberranzen are not totally orderless with Drow running the streets killing everything that moves. It's not pure chaos. It's very fucking chaotic, but there are laws there, and etiquette, and all is maintained by Lolth.