Let's talk about Sisters of BattleI picked up a squad of Battle Sisters in April, and borrowed a friend's Canoness for a 2000pt game against IG. The Battle Sister Squad destroyed a squadron of three Sentinels with Bolter Fire in a single turn of shooting (two immobilized, one destroyed), and the Canoness killed a PCS, and Infantry Squad, a tank, and an HW team. Since then, I've expanded my collection, and now have about 30 Battle Sisters, 5 Seraphim, a Canoness, a Dominion Squad, an Immolator, Exorcist, and Rhino.
Taking allied IG as SoB
This is something I haven't really done too much, being a Guard player first, foremost, and always, but I think it's worth covering both sides of the coin. The classic Sister list is a wave of Rhinos full of angry women with flamers and meltas zooming toward the enemy, usually supported by Exorcists and some Seraphim or Pen Engines. This list is very fast and very deadly, but is something of a glass cannon. If the Sisters get caught b a gunline, or bogged down in CC Swarms, they're pretty much done for.
That, ladies and gentlemen, is where the Guard comes in.
The IG packs heavy weapons like no one's business. As single infantry platoon can have up to 21 HWs if you go all out, but that really isn't necessary. Attaching a single platoon with 2 infantry squads and maybe 3-4 sections of HWs will really compliment a Sisters list nicely, giving them some ranged muscle to deal with the more deadly enemy units before the Rhinos arrive on scene with the Battle Sisters. a recent FAQ also states that if you take TWO IG platoons, you may take a squadron of Leman Russes. Yes. A Squadron. A basic Russ is always a good choice for this, but something nasty, like a Vanquisher to pop armour at range or a Demolisher to support the advancing Sisters, will really make your opponent's head spin.
Taking SoB allies as the IG
Ah, now we're cooking with napalm! Battle Sisters really add a mailed, 3+ save fist to the IG army. Two squads with Flamers or Meltas in Rhinos, supported by a solid gunline and some Vets or Penal Troops, will give you opponent a lot of very nasty things attacking him from all directions to worry about. Also, the psychological effect of a large number of 3+ saves popping up in an IG army is not to be underestimated. People have a tendency to panic and shoot the big scary women in Power Armour that they did expect (and thus have little AP 3 and 2 to counter them with), leaving your HWs, Vehicles, and Flankers to rip the enemy apart.
There are three units in particular that boost the IG nicely. Seraphim, Battle Sisters, and the Canoness.
Battle Sisters add that nice core of 3+ save troops to your line to use as a center piece, counter attack unit, or objective grabber. Sticking them in Rhinos gives you a very mobile, very evil unit that can run around lighting things on fire while your Gunline pounds away. You can also give the unit a Book of St. Lucius and Veteran Sister Superior and keep it in your gunline, essentially giving every unit withing 6in of that squad Ld. 9, unmodified. The downside is that they're a little expensive for an IG list, so only take one or two squads at 1500pts+.
The Canoness can do three things. First, she can be support. Giving her a book of St. Lucius and having her wander around behind your lines gives the same leadership bonus of 9 as that Sister Squad, but for less cost (Special thanks to Melissia on the IGMB for suggesting this). Option two is to tool her up for Close Combat and attach her to a PCS, combined squad, or CCS. This can be VERY nasty with Straken and a Canoness with a Blessed Weapon, which makes her hit at Str 6 at I 4. She also can absorb a lot of hits, making the unit harder to take down (especially if you tossed a Medic in there and gave it FnP). The third use, and by far my favorite, is the Flying Death Nun. Give the Canoness a Blessed Weapon, a Jump Pack, and Inferno pistol, and you're good to kill just about anything you run across. Tossing in a Mantle of Ophelia and Cloak of St. Aspira are also excellent choices, as they can give you a jump pack model that hits at Str 5, has a 2+ save (which can be made Invulnerable), and ignores the first instant death hit it takes.
Finally, there are Seraphim. These are nasty, nasty units when fighting with the Guard. A pair of hand flamers, a VSS with a book of St. Lucius, and Meltabombs will make this unit able to eat just about anything. Combined with Chimera Vets, Hellhounds, or Rhino Sisters, he enemy won't know what to shoot first. Their ability to close quickly, in addition to Hit and Run and being able to grab cover more easily then armour or convert their 3+ armour save into a Inv save makes them dangerous as all hell, especially when you attach the Flying Death Nun up there. I've got a dirty trick or two with Seraphim, but I'm going to keep quiet about what they are, lest I ruin the surprise for my opponents.