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.
No, Longinus does not know what is going on in the adventure as written. He is loyal to his brother to a fault, and this continues into adventure 11. Both brothers are old, but Pilus figured out how to slow down aging. Between being overly trusting and leaving well enough alone with the valley of Storms, he won't have anyone investigate. By the time people are disappearing, the monastery has battened down the hatches and is going to wait out the weirdness going on in Eresh.
Makes sense, thank you. I had Caela come to Seaquen in adventure 3 to oversee the mayhem, where she ran into the party, so I didn't want to involve her too much in the setup for adventure 5. Instead, Longinus asked the party via wind elementals to deal with the situation.
I was not sure how to make Pilus pay attention to the party but not prevent them from traveling to the Valley, so I decided to leave meeting him for later. Now I'll have to figure out what Longinus does and does not know...
Right, I see why it makes sense for gameplay, but in-world, if they are such trusty servants, seems like she'd use keep them nearby. I can invent my own explanation, of course, I'm just curious what the original story is.