You and I cannot affect everyone ourselves, however... Let me think. All other Sirens I know of are in Questing Country at present. We could teach others, but at this point that seems unlikely.

I believe most of their fresh water is via wells, which would be a poor choice for contamination.

I am having her gather all the holy texts so that we might look them over and make alterations as necessary, however. A show of containing and dealing with the threat ought to help content them. Particularly if we can reassure the rest of them that no other evil spirits reside among them.