Its a tough one. I would ask your sister if she and her husband still want to hear from him or not?
If they do:
Not much you can say other than let your sister know you had a good relationship with his mum who wanted to keep contact with you but he put a stop to that. You find it a double standard but it is what it is and leave it at that. Your sister will always take your side anyhow.
If they don't or are not bothered:
Then I would tell him straight, you find it a double standard that he wanted you out of his life, asked his mum not to stay in contact you yet you had a really good relationship with her, yet he is still bothering your sister and husband and should stop all communication with them
Frankly, he sounds like a complete ****. Don't get me wrong i can fully see why he wants his mum to not contact you: He will want mummy to invest her time into new/future girlfriend not you, that's how it works, however, you equally would want your sister to ultimately want to invest her time with your new man/future boyfriend. Really he should afford you the same courtesy if that's how he wants to play things.
This is a very challenging situation, all around the world there will be family / friends / friends of friends that may be known or connected to X's, was your X a friend of your sister or her husband before you met him? that would make it more challenging if it he was.
In all you are between a rock and a hard place, you cant stop people making contact with people but if it was me, if it was ****@ng me off, i would be prepared to call him out on it and point out to him his double standard.
Good Luck