What if there was a trait, "Losing the War" that all family members of faction x get if they lose 3 or 4 large battles in a row, with massive negative affects for moral, tax income, bribability etc. For balance there'd have to be some way to reverse fortune, I suppose.
If there only was a way to make the AI accept and honour peace, war weariness would be a good concept to implement, affected by battles/sieges/war length etc. There isn't .