According to BoLS at least:
I want to believe in Sisters getting a release this year, but I'm not going to hold my breath on it.40K: The Codices of 2015
The Army Books:
(release order is unknown, but the last rumors reported Necrons first)
- Dark Angels
- Chaos Marines
- Chaos Daemons
The "Special Armies"
This category are the new rumored "small codex armies" that have only a handful of units, short pagecount codices, and are entirely new to the game (or have been out of print for decades).
- Harlequins (rumors say Februrary more regularly)
- Others (conflicting chatter of everything from Ad-Mech to Genestealer Cult, and even more exotic factions...)
In general these are the "Imperial Knight" type releases this year - designed to make a big splash and spike sales due to the fact the faction is entirely new miniatures. Look for 1-2 of these this year.