That is genitive. In Danish you would say "familien Petersen" (the Petersen family) and not Petersens.Same model in Danish with the "s" at the end. Hos Petersens.
However, I never understood that as a plural. I'd see it as a genitive. Petersens (familie, hus, ...). Anyway, I haven't heard it used for a long, long time.
We don't do that in Danish. There aren't a lot of families like that left, anyway, so maybe that's why.Dear All, is it possible to refer to a whole family in Swedish/Danish/Norwegian, as we do in English (The Newmans) or in French (Les Sarkozy)? How do we form the plural in Swedish/Danish/Norwegian if we have the surname in singular?