This posting was researched and written by Janet Johnston of Pearson Education:
In Words Into Type (3/e, Prentice Hall, 1977, p. 479):
Proper names. The
possessive form of almost all proper names is formed by adding apostrophe
and -s to a singular or apostrophe alone to a plural.
Among the examples
listed are these two: "Shultz's" and Shultzes';
the first is the singular; the second is the plural. The next paragraph goes
Whenever the apostrophe and -s would make the word difficult to
pronounce, as when a sibilant occurs before the last syllable, the apostrophe
An example they state following that paragraph is Xerxes.
as in Xerxes' army.
This tells me that either of the versions of Sanchez that
Vera asks about is correct for the singular. If she wants the plural,
has to be
the second spelling. This also leads me to conclude that the same
rules apply as to any other proper name ending in a sibilant /s/z/:
just think of it as a different pronunciation of -s.