As mentioned, when you make a plural you never add an apostrophe.
Mr. Smith and his family, the Smiths
Mr. Byrnes and his family, the Byrneses
When you make the possessive from a word that ends in S, there are two different philosophies. Each publication will specify this in its own style guide. Neither has anything to do with the number of syllables or the accent on syllables.
If the word already ends in S, add only an apostrophe to make the possessive.
Mr. Byrnes’ car
The Byrneses’ houses
If the word is singular, add apostrophe S. If the word is plural, add only an apostrophe.
Mr. Byrnes’s car
The Byrneses’ house
Option 2 is the one specified by two or three companies I have worked for in their style guides, which is primarily for technical writing. If you pick up a newspaper or magazine, I expect you would find consistent use of one of this rules within the same publication.
When come back, bring cite.