How close were you, say 5 years ago?
The dh sounds like a horrible creep. It is really frightening to be harassed so I can see why you believe your sister should reply/contact you.
But seeing as this man is totally inappropriate with you re stalking, I cannot imagine what the hell is going on re his behaviour towards your sister. Have they got kids? Because he’s probably abusing her and it’s easier to get away if no kids. Honestly I think you should go round when he’s not there. Unannounced so she doesn’t have to lie to him.
If he was stalking you, that could be the least of his crimes. Have you got parents around? This guy sounds dangerous.