www.StudioColrouphobia.net, this guy's work is fantastic! He does artwork commissions as well, so definitely look into it if you have such a need.
This image made me think about how a Chaos Sister's army could be done. As far as rules wise, I suppose you could use the current Chaos Marines codex as counts as. It's really too bad there are no plastic sister's kits really to speak of. It really makes it difficult to do a large scale conversion like this. Sisters Repentia would be easy to cross over in to Chaos.
Found this as well, a great St Celestine re-imagined as chaos. If anyone has seen a Chaos Sisters army or has pictures, please share in the Comments below.