Creep: Who The Hell is Angela?

Something about the movie that always confused me is the phone call from "Joseph's" sister. In creep 2 he is going by Aron, the name of the victim from the first movie, so we can assume that Joseph is probably not his real name either. If that is the case, she would have to be keeping tabs on his aliases, or she is not his real sister, and she knows him as Joseph, but if that is the case why does she identify herself as his sister? If she knows he is dangerous, and clearly does not approve of what he does, why doesn't she do anything to stop him? Could she actually be an accomplice?

I know there is no definitive answer to any of these questions, but if anyone has a theory I would be interested to hear it.