The main subject is restrictive relative clause (no commas) and nonrestrictive relative clause (commas). A restrictive clause is essential for the meaning of the sentence. A nonrestrictive clause is information that is not essential for the meaning of the sentence.
In sentences 2 and 3, the clauses appear to be essential because they identify the boys being discussed. That means no commas (restrictive).
Number 1 is tricky. If the speaker has only one sister, the commas are needed because the information is not essential for meaning (nonrestrictive). If the speaker has more than one sister, the clause is essential to identify a particular sister, so no commas are used (restrictive).
Student or Learner