Where should the commas be in this sentence?
That was his sister Lucy's pet hamster.
Should there be a pair of commas surrounding ‘Lucy’s’? The possessive is throwing me off.
The sentence as written is perfectly fine.
That was his sister Lucy’s pet hamster.
Most style guides will suggest that you use commas for a nonessential appositive phrase, and leave them out if the appositive is essential. In this case, “Lucy” is technically essential if “he” has more than one sister (or the number of sisters is unknown), but nonessential if Lucy is his only sister (Grammarbook).
Because of the possessive, if you did add the commas, the rest of the sentence as written would not make sense if the phrase were removed (“that was his sister pet hamster”). You need to add a possessive to both apposed noun phrases if a nonessential phrase is offset by commas:
That was his sister’s, Lucy’s, pet hamster.
Related: Appositives with possessiveness