I can't properly understand this sentence
そして, そのマントは 他人には決して見えないことを, 彼女たちは知っていた.
I think it means:
And the girls knew, those cloaks (would reveal/showed) to strangers something they've never seen? And those cloaks (showed) something to strangers the girls knew they've never seen?
I want to understand this thoroughly. The lack of any verb in the first phrase is really throwing me off. Any help would be very much appreciated. :)