De-Essers are very useful for sibilance & reducing harshness. Often times it's common practice to place one De-esser on your vocal & ease up on the high frequencies; however, did you know a second De-esser on your chain will help with sibilance?

The higher range between 5-13k is the common area to target, but you should consider some of the upper midshipmen's as well like 2-4k.

Sibilance is usually more complicated than just higher frequencies. It's a complex problem & there are some lower frequencies that can cause problems too. 

More Tips and Resources...

Tutorials: Tips and Tutorials

Sample Packs: 
http://soundoracle.net/ 
Podcast:
http://bit.ly/YouTube-UnQuantizedPodcast 
Check out my gear on Kit:
https://kit.com/Sound_Oracle