I'd been hearing March as Chaos more recently... but I guess we'll see in the upcoming weeks, I suppose.
I looked at those Sisters of Eternal Mercy designs, and they look really nice. Even if GW does get SoB out in March/April, I may have to get some of those to add some diversity to my army.
From what I've heard today May is too soon, so expect this summer at the soonest for Sisters.
As for March, that is pretty much confirmed to be the last WFB End Times release. April is a bit up in the air right now because I don't know what could be fitting in there, but May is likely WFB 9th ed.
Ask not the EldarGal a question, for she will give you three answers, all of which are puns and terrifying to know. Back off man, I'm a feminist. Ia! Ia! Gloppal Snode!