My sibilants sound terribly sharp, piercing. Is there a simple effect I can use to reduce them

De-esser plugins are a cure, e.g. … Updated De-Clicker and new De-esser for speech - #199 by Trebor

A compressor set only to act above 4kHz will also de-ess.

Beware of headphones. It’s very common for headphones to exaggerate sibilance.

As you describe the sibilance as “terribly sharp”, then you probably do need to apply some corrective “de-essing”, but if you are monitoring with headphones, be sure to also check how it sounds on good quality speakers. otherwise you may over-correct and cause the recording to sound dull and muffled.