What to do about strange creepy admirer?

I work at my parents’ shop and almost every day there is this strange man that comes in. He can talk an ear off! He has one of those eerily super nice yet scary expression, voice, and manners. After he talks my ear off he always ALWAYS tells me to tell my oldest sister hi for him and how she’s doing.

My oldest sister used to hang around my parents’ shop a lot. When he first saw her I guess he developed a huge crush on her. A month ago he told me to give her a letter in an envelope. My sister didn’t know this guy and wanted nothing to do with him or this letter. I opened it myself and this guy is crazy!

He wrote in the ltter professing his undying love for her since the moment he saw her. Thinks about her every day for over a year, loves her with his heart, mind, and soul. He said “If I can not see you again to ask for your hand in marriage then I will do so through this letter!” He was begging for her to marry him.

A few weeks later (today) he came in again and gave ANOTHER envelope. It’s a card and it says “Happy Easter”....it’s no where near Easter. Inside he again, begged for my sister’s hand in marriage.

I’m honestly freaked out by this guy. I’m scared to tell him that my sister has a boyfriend and isn’t interested in fear that he might do something crazy out of rage and heart break. What do I do? I know I can not go to the cops because technically he didn’t do anything illegal.

I tell people this story and they think it’s funny but honestly it’s not funny I get scared. My sisters think that it’s innocent if they stay out of it…but I’m the one that is the messenger and the messenger always gets screwed!!