Kristen Stewart's Halloween Costume -- Faithful Girlfriend

Here's a picture of Kristen Stewart and Robert Pattinson holding hands on their way to a Halloween party at a cemetery in Hollywood last night ... but who cares 'cause LOOK AT LIBERTY ROSS!!!!

The "Snow White and the Huntsman" actress threw on her tightest, most revealing Catwoman catsuit to go trick-or-treating with her two kids last night.

Liberty's estranged husband Rupert Sanders joined the gang -- he was dressed up as a bad-decision maker who's desperately trying to win back his hot wife.