The plural would regularly be Hopkinses in English. If I invited your family over, I would say the Hopkinses are coming. If I was going to your family's house, I would say I'm going to the Hopkinses' house. If you specifically were giving me a ride in your car, I'd say we're taking Don Hopkins's car.

It's not a big mystery or even particularly complicated, all regular rules of pluralization and the possessive case. I think people get tripped up in school because they see a specific affectation of dropping the S from possessive forms of the names of some historical personages, e.g. "in Jesus' name."