I believe that sibilance can be best controlled at the recording stage with choice of mic, technique and vocal position. But if you're stuck with a track that has to be used, then I wouldn't suggest Logic's de-esser. Unlike most Logic plug-ins, I think this one is a dog. FWIW, I have heard good things about
Eiosis de-esser.
IMO, the best way is to painstakingly apply automation to the offending parts,or to actually edit the audio file, changing gain on the sibilant parts. I know, it's really time consuming, but I think you'll get a better result.