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Your parents could certainly file misdemeanor harrassment charges against them and allow the court to enter a temporary restraining order against them - this would prevent any contact (face to face, verbal (phone) or written). However, because it is your family, you may not want to go to this extreme.
Another option is to have your parents (or their attorney) write the siblings a letter indicating that their mom and dad are disgusted with their actions and if they do not cease their actions, a codicil (amendment) will be made to their wills excluding them as beneficiaries.
It is perfectly legal to intentionally exclude a child from a will (you can't do this to a spouse).
If that does not cool them off, then you may have to seek the restraining order.
Sorry for the delay. I stepped away for a few minutes.
Yes, your parents can do whatever they want to with the estate and give it to whomever they want!
All children probably have a sense of entitlement to their parents estate. However, when it comes to threats and harrassment, they may be overstepping their boundaries.
Their statements can certainly get them in trouble for harrassment. They could also be charged with libel and slander if the communication is published/told to the public.