So sorry you are dealing with this problem as I would agree that the former family that lived there might be trying to avoid bill collectors.
My first suggestion if you have not already done so would be to speak to the postmaster at your local post office to explain about the wrong address mail being delivered and that it needs to stop and to inform the mail carrier who delivers to your home each and every day. A postmaster finder link is below to help:
Another suggestion that was told to me by my mail carrier years ago when I was dealing with a similar issue is to block out the bar code that is on the envelope. Because many of the pieces of mail are automated when sent, crossing out any bar code that is found on the envelope can help in sending a message to the sender that the previous family no longer lives at this address.
Finally, another suggestion that I have linked to below talks about filling out a "change of address" for the mother/daughter.
I hope that these suggestion are found helpful to stop what is going on. Please let me know of any additional questions/concerns you might have as well as if you could take a second to rate my assistance so that I know my help was found useful today.