Like most professional publishers in the US we use the CHICAGO MANUAL OF STYLE. Sometimes it's hard for me, after years as a reporter on a daily newspaper using AP/UPI, to remember that Nouns ending in s, still get an apostrophe S to show possession. In the newspapers and in most English 101 classes that we all took in school, they just get an S and an apostrophe. But according to CHICAGO Only PLURAL possessives get the noun and apostrophe without the ending S. When they are plural, but not possesive, nouns ending in S get an added -es on the end.
Therefore the following sentence is correct, however inconsistent it may look.
The Graveses came to dinner at Thomas's parents' house.
Almost every publisher has its own list of exceptions to the CHICAGO rules, and if those guidelines are available on line, it's helpful for you to look at them and make sure your manuscript complies with their guidelines, before submitting it for publication, because even though editors and publishers realize that rulebooks differ, they also know that if they see "Graves' came to dinner at Thomas' parents house" in your manuscript they will have to go in and fix three mistakes.
And how do I know this? Because I've made every writing mistake there is to make at least once.