The subordinate clause here is - "who just finished the marathon". It is an adjective, a part of the subject "the woman who just finished the marathon". That subordinate clause describes the subject in more detail.
Subordinate clause: who just finished the marathon (ADJECTIVE CLAUSE).
A clause is a group of related words that have at least a subject and a verb. It may express a complete thought (independent clause) or it may not (dependent or subordinate clauses).
"who just finished the marathon" is a subordinate clause as it can not stand alone as a full sentence and have a coherent meaning. And it is also an adjective clause because it functions as an adjective: it describes or gives characteristics to the noun "the woman", and begins with a relative pronoun "who" (which is common in subordinate clauses).
They rely on me pretty much. I have an older brother so it's not that much but I do a lot for my family. I think that if they relied on me less I would never learn to be responsible for myself or anyone else.
Sentence fragments are groups of words that look like sentences, but really aren't. To be a sentence, groups of words need to have at least one independent clause. This however does not have a independent clause.