There has a been a lot of talk about the Sisters of Battle and what is going on with them. All the speculation and rumors cover the entire map of possibilities. If Sisters of Battle become a white dwarf army without a codex, it will truly be sad. However, the run of the mill is saying that this is just temperary, and that a full release is coming.
Don't forget the salt
Why WD update? Rumour is that the Necrons were delayed, they were scheduled for September but due to being pushed back there was now an empty 40k slot that needed filling. As the Sister were still in mid production and playtesting they decided to put the rules they had written so far into a WD split over 2 issues.
The WD rules may or may not be final but will most likely not contain the rules for any new units other than henchmen though most of them are just copy paste from the GKs updated versions of them (aka Death Cult, Arco, probably Crusaders and throw in the minor ordos as well with Hospitallers, Dialogus etc).
As for models I would not expect us to get anything Sister specific as that would be counter productive as that leaves no models for the real codex release early next year.
What we might get though are finecast Arcos, Death Cult and the rest of the GK Henchmen that may be used in both dexes.
A failcast Canoness or other HQ could be expected but I personally doubt it, there is no reason for GW to provide anything for the Sisters players yet.
As for when the real dex comes out you can expect plastic Sisters, Repentia, Priest and Penitent Engines. Probably some new kit as well or we might just get the a Sister specific Valkyrie or some such. Failcast Exorcist might rear an ugly head but there arn't really any rumours about this unit yet.
This is all we have so far to go on, we should get the first WD rules leaks at the end of next week or the week after. Until then all we can do is wait.