@Linda_D if her parents feel threatened, they could trot off and get a restraining order. They are a relatively cheap way of putting a stamp on someone's character (as your sister will have discovered) and it would not be an unreasonable thing to do if they fear further contact from him.
If he doesn't like it he'll have to defend himself; explain why he shouldn't be told to stay away from elderly people who are frightened of him. The only possible reason he would give is that he needs to contact them so they can see the kids.... If he accepts the order, it almost an admission their action is valid. These are shitty games, but he is already playing them
If he is as you describe, the insult to the ego will make him come out swinging. And likely make a complete pratt of himself
Involving the olds in a tit for tat has it's risks, particularly if someone is truly unstable. It could be a very dangerous thing to do on many levels. I'm a bit of an 'Art of War' fan, so this stuff does present as an option for a dark old soul like me.
I am in no way suggesting that this is the right course of action for your family. Sometimes its just nice to imagine watching someone trying to run through a field of rakes and getting repeated smacks for their stupidity.
