The Lannisters themselves have no claim. Technically, Cersei doesn't have a claim. Being married to the King does not put one in line for succession. The throne being so contested at this point means that the major houses would/should call for a Grand Council to decide who it should be. They would look at the family records and most likely decide who is the closest living relative to either the Baratheon line or the Targaryen line... Or I guess whoever has the strongest army could take it. That ain't the Lannisters either at this point.