In my attempts to set up a Sisters of Battle game using Only War rules, I found that no finagling of the standard kit will let you get light power armor using official rules, so I decided to try my hand at making up a homebrew Regiment Type for them (though given Sisters of Battle Orders, it's really just on a squad basis):

Regiment Type: Battle Sisters
Cost: 6 points
Characteristics: +3 Strength, +3 Toughness, -3 Intelligence
Starting Skills: Athletics
Starting Talents: Rapid Reload
Standard Kit: One boltgun and 4 clips (Main Weapon) per Player Character, one bolt pistol and 2 clips per Player Character, one suit of light power armor per Player Character, 2 frag and 2 krak grenades per Player Character.

I took Characteristics, Skills and Talents from both Line and Siege Infantry, as I figure the basic Battle Sisters would be expected to remain battlefield mobile even without Rhinos and be prepared to take up the brunt of the fighting at the front line. I waffled between 6 and 7 points for the cost, but since an 8 cost Regiment includes things like "Everyone gets a Tauros/Sentinel", I thought 6 was closer to the mark. Any feedback or suggestions would be appreciated.

You may wish to take a look at the work I did for my Sisters of Battle house rules. Save yourself some legwork, potentially.

Thanks for pointing me that way, Lazarus. However, I think I might want my ruleset to be more integrated with regimental mechanics, and I think I'd keep a lot of specialist positions (Dominion, Retributor, Superior) as Advanced Classes. I definitely agree with you on the Offense, Willpower and Ballistic Skill Aptitudes, and your Faith Aptitude is interesting, but the Wounds seem fairly high for what are ostensibly normal humans, and I think I'd go with +5 Willpower for the Battle Sister over the Ballistic Skill. I think the generic Sister build I had in mind gives them +11 to Willpower and Unshakable Faith, as they are meant to be elite warriors facing down rogue witches and heretics without flinching.