I been trying to find this movie for years, watched it as a teenager in the 2000s. There’s some sort of monster and they’re trying to find it, there’s a pale sickly man with long black hair who has a sister, he turns out to be the creature, he supposedly had one arm only but in one scene he takes of his clothes and he had the arm bound to his body to pretend. I’m not sure if the ‘monster’ was a wolf or something similar.
Out of curiosity, is this a silent movie by any chance?
Not a match about the one arm, but The Village (2004) has the rest as you described.
Could it be “Parasyte”?