Please note that I added in a second rumor (source) at the bottom. However I would say that the first is not only more reliable, but more a more accurate timetable. Rumors in the past have put the Sisters of Battle after the Sept/Oct release of the Necrons putting it early next year. Here is the latest and greatest on the Sisters of Battle.
Via StraightSilver
Well apparently the Sisters' plastics went Beta a couple of months ago.
That is to say test casts were done and sent back to the designers.
This doesn't mean that we will see them any time soon but it is progress. Jes Goodwin said that the Sisters project had hit a bit of a creative block, as what they wanted to do was causing problems. This was mainly the arms.
He said the DE Wyches taught them a lot, and that knowledge would be put to good use on the SOB plastics.
I would agree that Harry's estimate of a lead time of 18months is about right, so SOB plastics will probably be next year.
via Darnok
There will be a Codex: Adeptus Sororitas by the end of this year.
I trust my source, and it was named that way. But given the timeframe, things can change.