If the foreclosed-upon family member did record an easement sufficiently in advance the Sheriff's sale, then the new owners' title may be subject to a record easement which can be enforced by the rear property owner. If not, then you should consult an attorney concerning possible rights under a prescriptive easement which can arise if the access driveway was used for a period of time, among other requirements. Another type of right of way is an easement by necessity, but I doubt that exists since, as you mention, the driveway is not the only access.
If there are any easement rights in favor of the rear property, then a quiet title action may need to be filed to establish those rights conclusively. If there is a record easement, and the new owners block access, then you can bring an action for injunction (i.e. a court order directing the new owners not to block access).
This is not a simple question. To answer it, you should consult with an attorney, who would review the records in the Recorder's Office, and the surrounding circumstances, before offering an opinion. Two of the many questions are: Did the family member record a deed or easement? and Did the mortgage company release the driveway from the mortgage?
This is not intended to provide legal advice about your situation - just a few casual remarks about a legal question. We are a debt relief agency helping people file for bankruptcy.
I agree with my colleagues.
Your question presents complex legal issues. You should take the documents to a local real estate lawyer familiar with the law of easements to determine if you can enforce the easement.
A properly filed easement is generally enforceable.
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