if it was me, i'd back off of the relationship with his sister. you wouldn't even know about this otherwise and you would be happily focusing on your life and your recovery.
whether they choose to do an intervention or not, that is a matter for them to decide.
it does not have anything to do with you.
i would take nothing to do with it. you're absolutely right, it's too much focus back on him. and do you really want to drag your children through all that?
there is help available for him when he chooses it. if others want to force his hand, i would step back from those relationships and get on with my life.