This is something I have discussed with Jack previously in regards to the points and fees test. He pointed me to the Bank Holding Company Act, https://www.law.cornell.edu/uscode/text/12/1841.
12 USC 1841
(k) Affiliate.— For purposes of this chapter, the term “affiliate” means any company that controls, is controlled by, or is under common control with another company.
(a)(2) Any company has control over a bank or over any company if—
(A) the company directly or indirectly or acting through one or more other persons owns, controls, or has power to vote 25 per centum or more of any class of voting securities of the bank or company;
(B) the company controls in any manner the election of a majority of the directors or trustees of the bank or company