My sister who lives in Seattle is visiting next month.
Correct: A
Option A correctly uses commas to set off the non-restrictive clause 'who lives in Seattle.' A non-restrictive clause provides additional, non-essential information about the noun it modifies. If this information were removed, the meaning of the sentence would still be clear ('My sister is visiting next month'). Therefore, it must be enclosed in commas. Options B and C only use one comma, incorrectly separating the non-restrictive clause, while option D misplaces a comma.