We still can't be certain the Sisters are even going to warrant a release. The release schedule is there so that their sales is not overloaded filling out all the orders for the new release, but presently the Sisters codex is just in White Dwarf, not its own book. Until we get confirmation on new models, there's nothing to release, which leads me to this:
August: Necrons
September: Secret Special
October: Ogre Kingdoms
November: Sisters of Battle
My thinking is while we will get the Sisters codex in White Dwarf, new Sisters models won't show up until November. There is the possibility that Necrons and Sisters will still swap, I mean from a release schedule perspective it would make sense, Sisters is probably going to be a slightly smaller release, no codex, just a few models, and they will release Storm of Magic in July which will be a pretty big release. I'm still banking on Necrons in August though, though we really won't know for certain until someone gets August's White Dwarf.