Lordy, Lordy, Lontins are forty.
If the family name is Lontin and there's more than one of them, then add -s, like this, Lontins. As for when to use apostrophe s:
Ex: This is John Lontin's dog. <just one Lontin>
Ex: This is the Lontins' dog. <more than one Lontin>
Does that help?