As for Taru being duplicitous, I won’t put anything beyond her. Beyond any of the organisers. They specifically went looking for vulnerable people to recruit to a suicide mission, and chose their own family to do it!
It was rather "all in the family", wasn't it? Except for Mikkel. He really stands out as the only one with no apparent links to the organizers.
But, for all the catastrophizing, this doesn't seem like a suicide mission
initially. If the bridge had not been destroyed, they would have tooled around for a couple of weeks and then gone home. Looking at what actually happened in the first couple of weeks, it seems like they wouldn't have had too much trouble. There were the trolls in the first building and the dog-beast, but otherwise not a lot of danger. (Of course, I don't know exactly how long they tooled around before hitting the ghosts, but it was time enough for the kitten to grow up some.)
In terms of family connections:
Trond may have genuinely thought that Sigrun could handle whatever the Silent World threw at her. He was right, too. To defeat her took an infection, which would have been treated properly if they'd gone home as expected, and ghosts, whose existence no one suspected.
I don't think Taru chose the Hotakainens because of the family connection. When Lalli shows up at the pier, Tuuri tells him to meet their new boss, Taru, and adds that "We're actually related to her!" Taru says, "Not closely related, but sure." Tuuri didn't say, "Remember she used to visit us when we first came to Keuruu" or something like that, so I think it's just coincidence, and a measure of how small the surviving Finnish population really is, that the best mage Taru could find (Onni) was her second cousin once removed. Certainly there's nothing in the interactions of Taru and the Hotakainens that would have been any different if they'd been unrelated.
Finally there's the Västerströms, and yes, they specifically went looking for vulnerable people and they recruited Emil, who seems to be Torbjörn's only relative outside his brother (or is he ...?) and his children. Depending on your view of Torbjörn and Siv, this may be because they cynically didn't care if Emil died and figured they'd get his share if he did, or else Torbjörn didn't think this was a suicide mission (and I don't think it realistically looked like one), and he wanted his nephew to get a share of the wealth he expected.