Looking for a book i once read when I was in middle school

Hi everyone im looking for a book I had once read in middle school it was a really good book just to be a young adult book. Im 21 now and I've thought abiut it off and on. Im not sure anyone will know what im talking abiut as its been so long but I really wanna try. Young adult murder mystery book where girl finds out her cousin died and her cousins boyfriend did it. Main characters brother or maybe the cousins brothers name is Beau he may not be the brother and they thought the cousin and boyfriend kept hinting at mar1juan4 when they wrote MJ but was actually talking about the cousin whose name is like Mary Jane I think.
One piece of the book I remember the main character was with the the brother and they were in the murderers shed and found a pigs head or something? They live in a small town and everyone knows each other. Something in the town about crowning someone for some event in the town

The cover of the book is hazy in mind but colors were like white, black, green, pink. I think a girl was on the cover with a flower crown maybe or something?

I hope this is some what useful because I am STRUGGLING