The position that punishment should be proportional to how serious the offense is but that repeat offenders should receive harsher punishments than first-time offenders is unsustainable.
Conclusion (unsustainable = opinion)
It's not sustainable to think that punishment should be proportional to how serious the offense is and also think that repeat offenders should receive harsher punishments than first-time offenders.
Evidence
Were we to believe that, we'd be committed to thinking that what an offender did years ago (i.e. previous offenses) are relevant to how serious their current offense is.
If previous offenses from years ago were relevant, then almost every other remote consideration would be relevant, which would make it impossible to accurately figure out how serious an offense was so that we can assign a proportionate punishment.
Evaluate
If this were a Method of reasoning question, then we'd be saying that this author is undermining a position by showing that it's implications, if true, would be untenable.
The 2nd and 3rd sentence each begin by saying, . And the purpose of tracing out this chain of implications is to demonstrate that if someone believes both that "punishment = proportional to serious of offense" and that , then they're committed to the logic that a repeat offender doing action X is a more serious offense than a first time offender doing action X.
Given that it's the same action, how could the repeat offender's version of doing it be a more serious offense?
The logic would be that the repeat offenders' past actions make their present offense more serious, and that's where the author sees an impossible outcome, in which we have to assess all of an offender's past actions to determine the seriousness of their present offense.
Goal
We're being asked about the 2nd sentence, which was one of the three premises. It's the most direct implication the author mentions of the original position that she's refuting. From there, the argument goes through more implications until it hits the brick wall of "impossible" and thus proves her point that the original position was dumb.