Answers:
The prepositional phrase is: for the big family argument
The subject is: reason
The verb is: were
Explanation:
A prepositional phrase is a group of words (or a phrase) that contains a preposition and its object. So, in the example sentence above, the preposition is "for" and its object is the big family argument.
A subject is the one being talked about in a sentence. The subject in the sentence above is "reason." It is the one that is being dealt with.
"Were" is a "linking verb." Unlike ordinary verbs that show actions, the linking verb is used to connect the subject to its subject complement. The subject complement in the sentence above is "really pretty silly." It describes the subject "reason."