USCIS Policy Manual:
The interpreter must be impartial and able to interpret without bias. An impartial and unbiased individual is one who does not have a predisposition or preconceived opinion about a matter. A predisposition or preconceived opinion may prevent the individual from interpreting information accurately, literally and fully or making a reliable interpreter declaration.
Officers must consider potential conflicts of interest between an interviewee and his or her proposed interpreter, as well as any other circumstances that might interfere with the interpreter's ability to provide an accurate, literal, and full interpretation. Interpreters and the interviewees must disclose any relationship, predisposition, or preconceived opinion that could affect the interpreter's objectivity and consequently his or her ability to provide impartial and objective interpretation during the interview. For example, some friends, family members, or persons with financial connections to the interviewee (e.g. business partners) could have either actual conflicts of interest with the interviewee or have a strong personal interest in the interviewee obtaining the immigration benefit at issue such that the proposed interpreter is not able to provide impartial and unbiased interpretation services.9 As such, family members will generally be disfavored as interpreters if there is another qualified interpreter available to the customer. Upon disclosure, the officer must use his or her discretion in making a determination as to whether the circumstances will interfere with the interpreter's ability to interpret objectively and provide an accurate and truthful interpretation of the information conveyed during the interview. If the officer determines that despite the relationships or other circumstances disclosed, the interpreter can still provide competent, impartial and unbiased interpretation, then the interpreter may normally be accepted.10
Where the proposed interpreter is a derivative (e.g., spouse or child) of the interviewee and could obtain an immigration benefit if the interviewee's application or petition is granted, the officer should be particularly vigilant in making his or her determination as to whether the derivative may, nevertheless, be able to meet the impartiality and unbiased requirement. The officer should continue such vigilance throughout the interview for any signs that the interpreter is violating the interpreter's Declaration.11 However, the officer must not predetermine that a derivative beneficiary is disqualified from serving as an interpreter, due to a conflict of interest, solely because he or she is a derivative beneficiary.
Interpreters who are witnesses in the case-at-hand constitute a special subset of individuals who are likely to be inherently partial and biased, and, therefore, may be less likely to be able to provide accurate, literal and full interpretation. For this reason, witnesses are restricted from serving as interpreters. However, exceptions may be made at the discretion of the officer if there is good cause. See section (c), Restricted Individuals, and, Section (d), "Exceptions for Good Cause." As with any interpreter, a witness who is permitted to serve as an interpreter must demonstrate fluency, competency, and impartiality before and throughout the course of the interview.
Please note that some family derivatives may be witnesses, but not all derivatives are witnesses. Similarly, witnesses may also be non-derivatives. An exception for good cause is only required if the family derivative is also a witness in the case and the interviewee wishes to have his or her derivative interpret at the interview. However, officers must carefully consider whether the derivative is capable of interpreting impartially and without bias.