M, you've received some great advice here.
My adoptive mother has been cut off and cut direct for many years now. Like you, it was a combination of 'death by a thousand cuts' and one specific instance.
There was a history of big blowups and cut communication, but eventually she'd call like nothing ever happened, and weak me would recommence communications. Very typical behaviour for her - classic NPD with martyr-like ability to manipulate and guilt!
Eventually I polished up my spine, and after the last egregious incident, cut her off for good. I eventually moved to virtually the other side of the country and she no longer has any contact info for me. I kept my house in Other City for a year after I moved, and my sister, who was keeping an eye on it, said there'd been a flurry of phone messages left by Mother Dearest (sarcasm!) like nothing had happened. I told her 'delete and ignore'. Mind you, this was a good 3-4 years after the final blowup.
I am the oldest of 7 kids - they're all her bio kids - and thankfully none of them have questioned my decisions. Several of them wish they had the spine to do the same thing. She's getting worse as she ages, so they do keep me up to date (don't really want to know, but they need some moral support, poor things) and that's about it.
Anyway, after that novella... my point is, it's hard to maintain a cut, and the hardest time is when the cut is fresh. You want to scream at them 'leave me alone!' and tell them why. The thing is, why isn't important. They know you and the kids are OK. Anything else is just going to prolong the attempts to contact, and ramp up the hysteria as well.
Ask your friends on FB to block them or ignore. If you want to send a message via any avenue, just make it short and to the point - 'we're OK, leave us alone.' Block them yourself. Keep limited access to your sisters until things cool down. Ignore, ignore, ignore the parental units. Be the black hole!
If they go the 'missing person' route it will backfire on them. You're not missing - your friends know where you are. The police have way more important things to do and if they tried that here they'd be charged with making false complaints.
Last of all, ((((((hugs!!!)))))) I know how hard it is. But after everything else you've been through with your kids, girl I know you can do this. You do have a spine, it's in the right spot - it just needs a bit of a buff-up to get its shine back.