What do you think happened to the Sirens in the Stallionverse, and what do you think they'll do if they ever appear in the story?

They probably got banished to another universe AND a thousand years in the future by Starswirl, much like in canon (since, if they fed for a thousand years, they would be powerful enough where they wouldn't NEED Equestrian Magic. However, since Sunset never left Equestria in this universe, the Dazzlings have no Equestrian Magic to feed off of. Meaning that, if they tried anything TOO obvious, any moderately competent police force could handle them BEFORE they became a serious threat.