In English grammar, relative clauses are used to provide additional information about a noun without changing the meaning of the sentence. These clauses are often introduced by relative pronouns like "who," "which," or "that." When a relative clause is non-essential or non-restrictive (meaning the sentence would still make sense without it), it should be set off by commas. This is because non-restrictive clauses add extra information that isn't crucial to identifying the noun they describe.
In the given sentence options, the clause "who lives in Mumbai" is non-restrictive as it adds additional information about "My sister" but isn't necessary to identify who "My sister" is; therefore, it needs to be enclosed with commas:
This option correctly uses commas to set off the non-restrictive clause, while other options either place commas incorrectly or omit them entirely.