Originally Posted by AutumnalTone
Walk away from the parents, yes. As for the threat of abuse allegations, I'd also let them know I'd be responding by filing suit for defamation should they make false allegations. The best response to bullies is to let them know that you'll beat them down if they try to keep bullying.
There's more ways to make it clear to someone that you won't be bullied besides escalating threats.
I usually just call their bluff. Essentially standing firm that I will not let threats dictate my decisions.
But, I would take the time to discuss with them the likely consequence of their action - which, in this case, is the likelihood of losing the case as well as losing a connection to their daughter and grandchildren. Could I suppose be construed as a thread, but it's a reasonable prediction of an undesireable outcome.