If you know this man's identity then go to the Police and ask if he has a record. They may well intervene if they believe your sister to be at risk. However, once they have warned a person, they are powerless to do much else. After all, most affected are adults.
Many many years ago a friend met a bloke, I disliked him immediately - he put her down in front of everyone. Eventually she and I discussed his attitude, i.e. keeping her more and more isolated as well as verbally attacking her; and she had the courage to leave. I didn't see either of them after but I knew that she was safe.
I do wish that I had spoken out sooner
. If you are worried do not be afraid to visit the Police, they may be waiting for 'evidence' in order to proceed with 'stuff'. If not, write her a blunt letter and say that you have a bad feeling - however, if your hunch is correct, he may already be stopping her mail.