DNA
Strange they didn't just do a DNA test to determine if O'Brien was the son.
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What could Vincent have threatened Brandy and Mike with? They had only been setting up the fraud - nothing had yet been committed - no damage done.
And whatever charges MIGHT have been brought, they wouldn't have been in any comparison to being part of a murder!
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btw - Wouldn't O'Brien and Moore have been 'kissing cousins?' (if O'Brien had really been the son).