One sentence on these pages that BEST develops the idea that Isabella was in an unlikely profession and therefore had to prove herself to others is this: Since knocking on Eddie Boynton's dingy wooden door, she had helped bring in counterfeiters, bank robbers, and a hapless pair of blackmailers.
Why this sentence is selected
The claim is that Isabella was in an unlikely profession and therefore had to prove herself to others.
To achieve her goal, Isabella did not choose a worthy profession, rather she started helping the quest by bringing in counterfeiters, bank robbers, and a hapless pair of blackmailers. By engaging in these awful jobs she tried to prove herself to others.