I have been in the same situation, with the added stress of unwanted phone calls and family interference.
Slowly it tapered off; I blocked the phone numbers of all who were involved, then completely estranged from everyone.
The mail continued for a few years, my final reaction to it was - nothing.
I didn't mention it to anyone except my husband, I certainly did not contact my mother or her husband.
I just threw anything that came from them in the bin.
I know the dread you feel, I felt it for years.
The only suggestion I have- cut off their supply; no reaction, no mention to anyone of their actions and throw everything away, don't even bring it inside your home.
These people rely on a response to feed their drama, no matter the source, and word about you is fuel.
People here are coming from a place opposite ours, which is reflected in their responses.
I wish you peace, it has cost me my entire family except my own children and taken many years, but it's worth it.