In my game, I had Longinus be quite old and sick. I based him off King Baldwin from Kingdom of Heaven. So he's very weak and covers up his body. He hasn't been involved heavily in any business and mostly spends his days recovering, or more likely, slowly withering away. Pilus on the other hand has been studying lichdom and biomancy in effort to extend his life, hence his younger appearance.
Longinus trusts his brother, and Longinus is subtly manipulating things from the shadows, not wanting to 'bother his brother' with whatever issues they are dealing with. This allows him to go about his business without question. Also Pilus is using Caela, and isn't afraid to use her as a scapegoat. He has a really high bluff skill so its likely that he is able to string a web of deception around everyone and plan for ways to deflate any problems coming his way.