My ex-husband’s sister died — so why is Fidelity asking me for her death certificate?

Dear Quentin,

I recently received a letter from Fidelity asking if I had any information about someone they believed might be related to me. Apparently, she had an account with them. I wonder if this means I’m a beneficiary of her estate.

The person in question was my former sister-in-law. She passed away about five years ago. I was married to her brother, but we’ve been divorced for more than 15 years. She was never married and had no children. She has several living siblings who share her last name.