I don't know where this is from, but you have to consider that it might be wrong. I think the problem with using saw is with the "might’ve thought" part. It would sound better "if they saw Scotty’s older brother, they might think twice..." Perhaps the "saw" is there to indicate that they still have a chance to see him and change their decision.

This is not the subjunctive. The subjunctive employs the infinitive minus the 'to.' So a sentence employing the subjunctive would look like this: 'It is vital that he see a doctor.' You will notice that, in the above sentence, it looks like the verb has been conjugated incorrectly. After all we say (of a male cat) 'He sees well in the dark.' But the sentence about the cat is in the indicative mood. So the verb is conjugated in the way that are familiar with - I see, you see, we see, they see, but he sees. In most European languages the subjunctive occurs with sentences that involve doubt or urgency. In your sentence above it makes no real difference whether one says 'if they saw,' or 'if they had seen.'

Kyle, I think you are absolutely correct. In proper English, the text would read "because if they had seen Scotty's brother. . ." I would guess that the writers either didn't know proper English, or (more likely) they intentionally used sloppy grammar to make it sound colloquial or casual.