Neither Kavan or Baraz are a good outcome, and may represent distinctly different threats. Baraz is a military genius while it is increasingly likely that Kavan is a Mage slowly giving in to his worst instincts. If they gut each other's armies that may be all good, but we have no way of knowing what will happen. And aside from the Hastrijani we also need to deal with the Border Princes and the Nomads to consolidate the unternal politics of the realm. Diplomatic options are thus limited relative to our needs.
That said, Valda will almost certainly help, after a fashion, in the face of a military threat from Hastrijan.
And yeah, well. Backing the peasant revolt is high-risk, high-gain. Religious revolution frequently is. It looks likely that the Sylltaji are a more egalitarian faith than Amalism, and civil war breeds the destruction of the small folk. That kind of churn might be the perfect environment for an upending of the social order and conversion from below. But the peasants might not bite and the upper classes would be furiously resistant.
That said I get the impression that everyone uniting will not happen. Kavan is too far gone for it and Baraz will not be walking back accusations of parricide. The nobility would fight, though, and if Kavan or Baraz were to be killed their troops would rally to the resistance.
The twins are a safer option for intervention. They enjoy a lot of elite support and have an army and a base of territory. They are likely insincere about their willingness to convert, or at most superficial about it. I suspect the Sylltaji frown on incest, to put it mildly. But they are likely to be kind of absorbed and prove to be far less of a threat if they win. And they have not entirely unreasonable chances of doing so, especially if Kavan is put down and Baraz suffers a "terrible accident."
Valda might not forgive the accident, though. I increasingly get the feeling there's something personal between the two she wants to settle herself.
Though contact with the twins may still be useful even if we don't intervene. Offering them sanctuary would put a pretender in Illona's arsenal for later. Marriage to a weak pretender entirely in Illona's power in order to produce an heir with a claim on the Hastrijani throne might be even more useful.