HA HA i have found a way to use Sister of Battle models in the realm of the great crusade and heresy....there is a bunch of cards put out in the dark millinium card game called the sisters of silence, psychic nulls, used by the emperor as guards and psyker hunters here is a link to the 40k wiki

http://wh40k.lexicanum.com/wiki/Sisters_of_Silence

but still there it is, a way to use the witch hunter models in our themed army's, of course they need to be used under allies rules, count as psychic nulls, and count as elites and can only take one squad

hehehe, sorry to tell you this but its all ready been done. check (UK) WD 332 for details, but basically it was an army for the doubles tournament.

NNNOOOOOOOOO!!!!!!!! WHY I AM ALWAYS THERE SECOND...FETH U WHO EVER U ARE who...i ....am...yeah..* sighs * it sucks...i thought i be cool and find that...but of course i am not...the story of my life * crys * i am gonna go curl up and try to sew me a plush marine then cry into it

if youre really set on teh idea, the article didnt really explore the sisters that much it focused on the custodes.

so maybe you could try and come up with an army list for them. ie, suggest what restrictions they should have and so on and so forth