The Next of Kin form is used if the estate of the registrant is settled.  In that case, the refund is issued in the name of the next of kin.
What should I do if I decide to transfer ownership of my recently deceased mother's car?
You can use PDF [2} MV-843 as the checklist to help you transfer the ownership of the vehicle.
Since my mother has passed, who can issue a refund to her estate for her registration?