It's a
GW rule.
It isn't clear.
Personally, I could care less ruleswise.
It does seem to go directly against the fluff of Sisters, who are about as anti-chaos/psyker as grey knights are.
Without the crappy ruleset of the
DH, of course.
That said, I don't believe it's been
FAQ'd so you must bring a
DH inquisitor...so you can.