I know you italicize words in other languages when writing. But what if you add a contraction? Is it abuela's or abuela's?
It's really a matter of style. Most style guides don't want you to use contractions (e.g. 's for is), so you'd resolve this by not using a contraction. If it's not actually a contraction but a possessive then the style guides I've seen suggest not italicizing the apostrophe + s. For example:
Possessives [of foreign words] are often formed in the English manner, too:
The chador's folds concealed her form.
The Chicago Manual of Style practice—making the apostrophe and s roman—seems more acceptable here [than when forming a plural, as mentioned in a previous paragraph]; because of the separation provided by the apostrophe, the distinction is more apparent in printed type and less awkward-looking in typewriter underlining. But the same objections can be made by purists—a foreign word has been given an English inflection.
I advise avoiding use of English plural and possessive inflections for italicized foreign words, but allowing an occasional exception to avoid awkward rephrasing. The lapse is a minor one.
The Handbook of Good English
This is purely stylistic as far as I know, but I would use "abuela's".