The Scarlet Brotherhood was about the only plot hook my players didn't bite at. I think I portrayed them as too little of a threat. They didn't readily seek out the representative from Sasserine, as the other sidequests may have struck them as more pertinent. Same went for the Scarlet Brotherhood spies/monks/pirates on the isle near Farshore.
That said, the Scarlet Brotherhood is an interesting enemy. Surely you could portray them as insidious, by consequentially making them appear relatively harmless. However, if your party is ready to bite, I wouldn't let'em down.
Because of the Scarlet Brotherhood's propensity toward "racial purity" (read: bigotry), you may want to make the sidequest primarily one where the enemies the party faces are all humans with class levels. My rule of thumb is to come up with something atypical to a series of encounters with just some monks. Arguably, the stat block on p.35 in Dungeon 143 should be handy for representing the more standard monks present. Since the description of the isle (unnamed) indicates that there are only four of them, you may choose to either expand how many are present, or have only one or two of the monk/clerics use this stat block. Consider a human paragon (Unearthed Arcana) with levels in crusader (Tome of Battle) as the leader, or even a warlock (Complete Arcane) with levels in hellfire adept (Fiendish Codex II: Tyrants of the Nine Hells), since the article claims they worship Pyremius, god of fire, poison, and murder, and are themselves lawful evil.
Also, if you plan on sticking just to humans with class levels, you should give them a strong fort with lots of cover, and several crude traps, such as jutting poisoned bamboo spikes from the floors, or trap doors dropping into tar pits.
As for roping the PCs into visiting them, maybe the Scarlet Brotherhood invites them for a dinner, planning on poisoning their guests with a poison with no initial damage, but secondary...say unconsciousness. And maybe they are to be sacrificed over an altar for their fire god...
Hmm...maybe it's not to late to run a sidequest of my own...
EDIT: Whoops! Guess they already dealt with some. Well, maybe not all of them, right?
In my group one players was a member to scarlet brotherhood so when it came preparing Farshore for the upcoming pirate attack, he convince the mayor (Lavina end up losing the election) that brotherhood could offer their add in the fight. The look on some other players faces when the monks came off the dock was just priceless. The Brotherhood was more than willing help Farshore for chance to bring under their influence.
Suloise is the language of the Suel, whose empire centered on the area that is now the Sea of Dust. It was wiped out by the Rain of Colorless Fire, but many refugees made it to the Flanaess, among them the founders of Sasserine and the forerunners of the Scarlet Brotherhood. Several major deities in GH, notably Wee Jas, are from the Suloise pantheon, and the Suel were noted for their achievement in the arcane arts.
The Scarlet Brotherhood, though, is an isolated and idiosyncratic branch of the Suel race, their society having evolved in the isolated Tilvanot Peninsula. They have a strictly hierarchical society with a definite sense of themselves as a sort of master race, and they train those agents who leave the homeland to be fanatics, so the "Aryan" idea is not completely off base.
Too get a more nuanced picture, I recommend you pay the $4 to get the pdf file of the old "Scarlet Brotherhood" book put out by TSR, available from Paizo. It's got lots of great info on the Amedio Jungle and the lands to the south of the Flanaess, so it's great background generally for running the Savage Tide AP.