It's likely not cyberstalking, but you should have an attorney send him a cease and desist letter. That might get his attention to leave you alone. Block his numbers and email accounts.
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If you can, you and your husband should cut off all forms of communication with him.
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
No. Under California Code of Civil Procedure section 425.16, such ranting and raving is arguably protected speech as part of one's First Amendment Right to Petition. Were you to sue your relative for such e-mails, you might be met with an "Anti-SLAPP" motion filed by counsel for your relative, which if granted, could subject you to significant legal fees. The First Amendment protects such speech, as crazy as this may sound, further under the Litigation Privileges which is codified at Civil Code section 47 (it may be CCP section 47). Be careful about stirring up the pot on this. Mr. Harris' advice to cut off all contact is wise indeed, regardless of the legal reasons I state above.