What I'm hearing is almost a tight room sound,, like the voice was recorded in a tile box. This is what's going wrong. The sibilance almost sounds like it's a reflection off a hard surface. 
 
I don't know how that was recorded but it's way off as far as warmth and tone go. 
I would re do it and use a different space , mike, pre amp. 

Hrm, not hard surfaces, actually custom made baffles around the vox area. Did the whole record like that and I'm four tracks in and vox sound great on everything else. It was a loud track though, so I think we were too close and too hot. 

 If I didn't no better I would say you have put effects on the voice. Like I say it sounds like a tile chamber to me. The sibilance almost sounds like it is triggered by a reflected sound. Like the room had a frequency spike. 
Was there a music stand? A snare drum? 
 
It got me thinking it could be caused by headphone leakage. Or some sort of feedback loop in your monitoring. But there's defiantly a 8-15 ms delay in there. 
Either that or the MP3 encoding is weird. 
 
Other possible causes could be overdriving your A/D as that sound is not what I call a normal sibilance artifact. It's acoustically or electronically induced. 

If it were headphone leakage we'd hear the music in the track, no? 
 
There was a music stand involved, no drums though. I'll double check my export and make sure it was fully dry.
 

Whoops, yeah you were right, left the doubler on (how embarassing)
 
Original link updated, but it's repeated here:
https://soundcloud.com/fogpeople/vox-sample
 

I take it melodyne didn't work then ?

Nope. At least not to my own skill level at it. I just tried doing amplitude changes...is there a special spectrum tool I'm unaware of?
 

Well that's much better, I knew there was something more than what you were describing. 
But first observation, I certainly have trouble hearing what I'd call sibilance now. The doubling made it much worse so I would not add that effect at this point. 
I see there is at least 3 places in the clip where "S" stands out but in a full mix I can't see this being noticed. I hope you not just going with soloing the vocal track and hoping it's perfect. I would go with a full mix first and see if others notice it. Your going to hear it now until you die!! :) 
 
I opened it in Wave Lab and I applied a hi pass at 7,000 with a +7 bump at 1,200 which was to add what will be lost in volume. . I focused in on 
"It's a sideways glance "   then    "it's never touched"   and just rendered those 2 places. 
And it did improve the sibilance a bit. Probably enough to make the grade. Sorry I can't post a sample but just try it yourself. You just need to zoom way in and isolate the "C" or "S"
EQ will certainly diminish the issue but in these cases an automated solution will probably do more damage than good. Manual editing a vocal track is my go to solution for these sort of issues. 
While your in there you can fix all sorts of little things. I use narrow EQ filtering to get rid of sibilance and Plosives. You won't notice a filter on one little 1/2 second syllable like you will on the whole track. 
It's also standard for me to look for quiet passages and use gain to level things out. Just a little here and there does it... you can't overdo these things. A gentle touch. 
Another observation:  
Her enunciation is sort of the problem like she was not exactly sure of the lyrics and is sort of slurring those parts. I found it real hard to understand what was being sung.. but that a good thing in rock and roll..  

Wow, thanks! I'll dig in and try and recreate.
 
Just for clarity, you say a hi-pass at 7K, did you mean a low-pass?
 

Ah, no, I see now. Yes this works well! Okee, I think I'm back on the case here. Thanks so much!
 
I'll chime back in when I finish with the vox and y'all can grade my work. :-)