I agree with @Zoffiel - writing a letter is a good thing - years ago I had a SIL, of the classic bitch variety, and I was so hurt by her nonsense that I sat and wrote and wrote and wrote and then I re-read it and re-read it and after a few days came to the conclusion that she really was a classic bitch that won't change and I don't need her in my life and hence threw the letter in the fire and felt better for it!
All these years later, 10 to be exact, I saw her at a family funeral and yes she hasn't changed, sits up like a Queen expecting all and sundry to visit and hold court only about herself..........to my knowledge she has no idea about my BC