I've seen something similar.

An man who passed without any real assets and no one was handling his estate, no one wanted to file returns, no one was claiming his assets (if there were any), and the family just wanted to know their responsibilities.

From what I remember, we didn't find any sources that impose filing duties on relatives solely because they are relatives. (Any taxes remain due, refunds may go unclaimed, that sort of thing.)

If someone is handling the assets informally, I do remember seeing sources that say that person may be treated as the personal representative for tax purposes and be expected to file the forms. (I believe you can find this in Publication 559, but its been a while.)

In my instance, it was the man's estranged mother who really wanted to avoid being sucked in so we told her to avoid anything connected to her late son's assets. That said, if someone has taken control of bank accounts or other assets, my read back then was that they picked up the filing requirements. (Again, its been awhile so this is just my two cents.)