In the case of a surname ending in "s", people do sometimes add "-es" to talk about a family, collectively. However, it is not very common to do this. I think this form is used in your example to give a bit more impact/ emphasis to the final exclamation. I'm assuming this is an advertisement, or something of the sort... Usually, you would just say 'the Williams'.

I think this is something that even many English speakers make mistakes with, because there aren’t that many last names that end in an S sound. But the “rule” is just like with any word that ends in an S in singular. Kiss- singular Kisses - plural