We actually know why. There is a scene with Ella meeting up with Langdon, 5 days after MC kills Klaus.
It becomes clear that Ella traded her services for help with finding the black stone she lost during the fight with Alexis and Clark.
We also know that Jared is tasked with searching for the rest of them, which is probably why Ella was willing to work with Langdon indefinitely as he has the means and the will to do it.
Langdon already knew about monsters for a while, probably for more than a year. Xanthe was working with Dietz, who is a member of Langdon's cabal. It's how Klaus got his powers in the first place and bringing in Danica to train him probably meant that they already had their feet wet for more than that.
Based on what Langdon says during that meeting with Ella, it seems to me that Dietz was the one involved with the Monster side of the cabal's operations and losing the superhuman trio (Klaus, Danica and Oscar) was a huge dent to their plans. So Langdon decided to take matters into his own hands. He basically obliterated the Aldain syndicate, forced them all to turn into Superhumans with Ella's pills, without regard for their compatibility.
The kids themselves didn't even know they were part of SIN at first. Shen calls it "Cole's dumb assignment" during the Diamond Battle.
However SIN seems to have evolved in a sense, mainly because Tanos is heavily involved (who seems like a recent acquisition, I don't think he was there from the start). However we don't know his circumstances. The certain thing is that only under Tanos direction SIN has some legitimacy. The kids may be able to sway monsters like Hex and Goliath assuming that a stronger Superhuman can subdue them, but I doubt anyone there, including Ella, is capable of dealing with the Chosen.
Tanos is the son of a Chosen and has a good relationship with at least 2 of them.
I don't think she cares about the "invaders". Her plans involve the Apostles, a ritual and an extremely powerful Superhuman who ultimately will decide how this whole thing will play out. The question is what she wants to get out of it.
1) Ella can communicate with an Apostle. During her fifth evolution, Ella urges the Apostle to go back to sleep saying that she will take care of stuff.
2) The Apostle is most likely Memory. She is the only Apostle who actively wants a 50-50 Spawn, she is the one that Ella brings MC to during deadend3.
An interesting question is why Ella considered both Jake and MC adequate for her plans.
Does that mean that Memory does not care if the Spawn is of her own line?
Or does it mean that Ella wants to "hijack" whatever Memory wants to do and turn it to her advantage?
Considering the deadend3, I'm willing to bet that up till now, everything is part of Memory's plan. This plan mostly works for Ella, but she will probably have a contingency in place for the end.
