Okay, so the likely move and countermove here is that Abadar bans Black Sinflower or possibly Sinflower more generally, Urgathoa continues producing it anyway and smuggling it. There'll be a demand, at first because it gives an intense high, then because people are addicted.
Urgathoa's put a lot of effort into this one. She must have spent quite a bit to pervert the inherently positive-energy based nature of Sinflower to instead run off of negative energy. Then again, Urgathoa is the patron of undead: making things run on negative energy that should run on positive energy is kind of her thing.
Sunaira's first option for her defense is to introduce some other form of drug - either a medical treatment that helps with addiction or a safer alternative to Black Sinflower that still gives its characteristic high. She has the second already available, but releasing it feels like directly playing into Urgathoa's hands. The endless quest for a purer and stronger variant of Sinflower is Urgathoa's domain, not hers.
A step-down drug is safer. She has such things in her collection as well - they're part of her domain of sacrifice - but again, Urgathoa is her better in the realm of drugs.
If she's going to introduce that, she'll need real backing from a deity of healing who can contest Urgathoa on her own turf. Sarenrae or Yeulral.
What was Urgathoa doing already in the realm of drugs? That matters to her assessment; sad as it is, if this is just one more tool in Urgathoa's large arsenal of narcotics and intoxicants, it might not be worth fighting specifically. It might be better to fall back on domains of community and support and help people with the problems that make them turn to Urgathoa rather than try to individually stamp out every new drug she makes.