According to Scans from WD rule book the Sisters are listed as an official army in 6th edition meaning they are hopefully not discontinuing the army as some feared and giving some hope that a new codex and new models are indeed in the work.
The sisters will be one of 7 armies with no access to any of the new psychic schools along side Black Templar, Chaos Demons, Tau, Dark Eldar, Necrons and Orcs. Hopefully this means they'll be issuing a new codex and fixing/expanding on the current lack luster Faith System to bring it on par with the newly expanded psychic powers.
Hopefully though the Sisters will be able to ally with any imperial forces so we'll be able to pick up some psychic units that way. Lets hope with the new ally rules sisters start showing up more regularly in games to show GW the army has potential. If the new rules don't adversely affect Celestine it may well be worth taking the sisters as allies just to take her as an HQ choice even if you have to take one squad of battle sisters too.
Sounds like the old faith system might have been combined with psychic powers. Not sure what that might mean for the Sisters. It could just mean that Faith will now be a variant psychic system or maybe that faith will be something more exotic in 6th. Regardless I hope they scrap the existing system since it doesn't scale, adds little strategy, doesn't really make for any interesting army building options, doesn't interact with any other mechanics and isn't really very reliable compared to psychic powers, among other issues. I think a clean slate with a mechanic tailored to 6th edition could be a huge improvement over what the band-aid codex has done to faith.
Of course it could just mean they are not really thinking much about the Sisters and Faith right now too but I prefer to be optimistic.