Heather, who is Chuck's sister, sews and loves making clothes for people. In this sentence, what type of pronoun is "who"?
Who is a non restrictive, relative pronoun which introduces the relative clause "who is Chuck's sister".
Who is Chuck's sister -- is a dependent clause which "relates" information about its antecedent, Heather.
If we omit the clause then the main clause ( Heather sews and love making clothes for people) meaning will not change as we know is he? Heather. Or it is not an essential clause for the sentence. It brings about a little of more information only. This clause we can say an appositive too.