The response of your microphone and your relation to it can effect sibilance. With most cardioid microphones, getting closer to the microphone will increase proximity effect and provide a natural boost in the low end. And moving a little bit more off or on axis may change the frequency response. So you might want to try some different mic techniques. Of course different mics also have different responses and a mic with a response that naturally increases the level at the frequencies where your sibilance is occurring will make things worse, so if it is an option then trying some different microphones might also be beneficial.

As far as other options, some digital mixers make de-essing, equalization, gating, compression, etc. available for each channel, great for not only addressing sibilance but also for creating various vocal effects. If your mixer does not have any of that then you could spend anywhere from a couple of hundred dollars to several thousand dollars on a 'channel strip' or vocal processor that provides similar processing. Of course, if it is all you have available to you then some judicious channel equalization may help.

Thanks for the suggestions! Will give them a try.