How can I report someone hiding my mail and sometimes tampering with it and ripping it completly?

Anonymous
I've been looking online but I can't find the answer I'm looking for.I don't live at the mail address anymore but my grandma and uncle do they give me my mail when they can but alotta times my cousin which lives nextxoor on the same property takes hides and sometimes opens and ripped the mail. She not only does it to my mail she does it to my grandmother..I would get a PO box but I can't aforrd it with my current income.we've tried reasoning with her but she absolutely hates me and my grandma.what can I do legally to warn her to stop because wverytime I try bringing in the police she makes things up and the police treat me like the bad guy somebody help me its not fair that she's doing this to me and my grandma
Updates
+1 y
I can't aforesaid a P.O box and were I live I can't receive mail I donthave friends that I trust enoughto send them my bank satatements and other very important letters..also I would record her but I live in California. And it's illegal to record someone without there concent ceven if they do live in your house.would it be possible for me to get the post office to send a letter about the consequences of stealing Mail can one of you maybe help me write a fake letter like this?
Updates
+1 y
Sorry about the typos
How can I report someone hiding my mail and sometimes tampering with it and ripping it completly?
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