Oh yea, investments. That's a third way where family wealth can directly help in a war effort, along with calling in family aid and buying regions.
The possibility to tax/raid/interact with family homes more would be interesting, but would need proper thought.
I'm not sure we really want to force people to end ongoing wars through fatigue mechanics. Ideally, yea, more shorter wars sound fun, but inertia is so huge in this game, and it's so hard to get a war going... that you really don't want to end it sooner than it has to, because making the war shorter won't necessarily make the inter-war peace any shorter.
You are talking about the same inertia that drives people to fight until they demolish a realm.
