Generally speaking, you're only legally liable for a medical bill that belongs to you personally or your children. But if you did ever sign for emergency care for someone at the hospital, even if it was a kid down the street, etc, you can be held legally liable.

Fortunately, the hospital couldn't find any paper they claim you signed so your problem is resolved. IF you had signed for it, it would just be a matter or having your mom go to the hospital business office and absolving you of the responsibility by signing for it herself.