I wouldn't be at all happy about this, especially if what you know of this woman isn't good. My DS has returned several official-looking letters to a bank recently, clearly marking on them that the addressee was not known there. Still they kept on coming. Fortunately, my little DGS tore the last one open. It's a very serious demand for a substantial sum owed by this unknown person to the bank. Without knowing that, how could my DS ever have begun to sort this out?
Parents-in-Law. What do/did you call them?