The easiest way to explain the twins is that neither can succeed without the other. If you remove Jezera from the situation then Andras fizzles out pretty quickly because he doesn't have the necessary interpersonal skills, connections, knowledge, or magical know how to conquer the kingdoms of man. Jezera also fizzles out pretty quickly if you remove Andras because she doesn't have the necessary strategic know how to lead a military from the front lines, or the brutish charisma needed to inspire an army of orcs, and she's far too impatient and childish to properly wage a ground war. Without each other they're a regional warlord that no one takes too seriously. With each other they're a slightly larger pair of warlords who are eventually crushed by the nearest kingdom when they get a little to big for their britches. Whatever skills and traits they might have that make them effective are negated by their lack of key attributes that they both lack, they forcibly recruited Rowan and use magic to keep him under control for a reason.
Rowan is the only thing that makes the twins an actual threat. Before he came along they were housed in a tiny, decrepit castle in the middle of nowhere with no standing army and without the necessary funding to build one. Rowan singlehandedly recruits a country threatening armada of orcs, monsters, and men, builds Castle Bloodmeen up so it can support the people necessary for the war effort, and is able to manage the twins effectively enough to keep their bloodthirsty, impatient, and childish behavior from ruining things.
tldr; Neither twin is the smart twin, they're both too impulsive, too dimwitted, and too shortsighted to win any kind of war.
