English teacher here.
We have some serious exception-taking to the rules going on - but that's fairly typical of English, as well as a delightful schism between American and British forms, also fairly (and increasingly) common:
Nouns (not including names): Plural nouns that end in s (like “guys”)
don’t add another s to form the possessive, e.g., the students’ lounge. “The students’ apartment” is the standard spelling. (Chicago Manual of Style, American English)
First names: Chicago style adds an apostrophe
and an s: James’s, Iris’s. But please see CMS 7.17–23 for more examples and exceptions to the rule.
At this point it devolves into: "Q. When indicating possession of a word that ends in s, is it correct to repeat the s after using an apostrophe? For example, which is correct: “Dickens’ novel” or “Dickens’s novel”?
A. Either is correct, though CMS 15 recommends the latter. Please consult 7.18–22 for a full discussion of the rules for forming the possessive of proper nouns, including exceptions and examples. For a simpler statement of the rule, see paragraph 5.26. For a discussion of the alternative practice of simply adding an apostrophe to form the possessive of proper nouns ending in s, see paragraph 7.23."
For my personal preference: Ennis's and Jack's families, Ennis and Jack's love for each other, Jack kissed Ennis on the Del Mars' back stairs, Jack rode Ennis's <horse>, etc.
If you care to read a British article (none the more helpful, full of contradictions and exceptions), but still very very funny
http://www.worldwidewords.org/articles/apostrophe.htm
-backtobrokeback
