Q: My younger sister has been persistently harassing me for years. She is now threatening to go to tax authorities

A: Your first step is to send her a written demand to cease and desist from all such harassing conduct, describing with some specificity the types of actions that must stop immediately, and stating that otherwise, you will be forced to take legal action. (You can also demand that she cease communicating with you, period.) You also can have an attorney send such a letter (which is not necessary for that step but could be more intimidating.) If she then continues, that are various legal steps you can take, depending on the action. Bombarding you with emails or texts against your will after you demanding that it cease is cyber stalking, for which you may petition for an injunction against domestic violence (commonly referred to as a restraining order) - and you don't necessarily need an attorney for that; a large percentage of domestic violence court petitioners represent themselves. Regarding the threats to report you to the IRS, your inquiry is vague as to her intent. If her intent is to extort you (demanding something for her benefit and telling you she will report you if you don't comply) would probably be the crime of extortion, for which she could be arrested and prosecuted. Another potential issue in your situation is intentional infliction of emotional distress, for which money damages potentially may be sought - but that cause of action has a very high threshold to meet for it to be a viable claim.