Your BIL is a bully.
He has bullied you and others by putting you down and silencing you - but he still had the emotional energy of your anxious presence to feed off. Now you have removed yourself from his orbit this will send him into Narc rage. He will be out for revenge - looking to punish you.
The BIL is rattling your cage - he is desperate to provoke you into a reaction so that he can twist and use it against you.
That’s his toxic fuel, his oxygen essential for his survival.
Don’t give it to him.
That’s the most powerful thing you can do.
It will liberate you and frustrate him.
Don’t react, engage or comment to him or anyone else. Give zero info about your lives to any of his flying monkeys. Go NC / LC with the lot of them, cut back your SM.
Know that your detachment, indifference and distance will sting him.
Don’t worry about smearing. This has happened to me - the advice I was given was that you can never get ahead of the bully in trying to clear your name - it’s a futile, exhausting, wild goose chase and your involvement in it is inadvertently fanning the flames. It’s a trap.
Be confident that you have your own reputation built up over decades and he has his.
Trust and believe that those that matter don’t care and those that care what he has to say don’t matter.
Your DH can go to stuff if he wants - but you should model to him what “emotionally taking care of yourself” looks like - ie calmly taking yourself out of punching distance, dropping the rope on toxic games and swerving the provocation and conflict of difficult people.
He seems to think you have to tolerate bullying and abuse - because that’s what his family system was all about. If you calmly have more kindness and respect for yourself by removing yourself from this he will see the benefits and learn to do the same.
The echo chambers are enablers of his behaviour.
Are their children / cousins involved in this?