Paying her dues
After years of this treatment, of neglecting her and taking her money, the Banks had the audacity to tell Sam that “she owed them” for taking her in, and that when she turns 18 and gets hold of her money, she should remember that and be generous towards them.
Whether they believed it or not, they told Sam she should give them some of her money, as a token of appreciation for all the hard work they put in taking care of her all these years. As Sam was approaching her 18th birthday, the Banks reminded her of this constantly.