In opposite to authorized medical interpreters, relatives are not (generally) skilled in the medical terminology that might be needed to convey a message from a healthcare professional. In addition, the level of their general language ability is unknown. This may lead to misunderstandings and inaccurate interpretations.
Children are especially unfit to interpret. They may not have the ability to understand adult issues or communicate critical personal information. But most importantly, they run the risk of being traumatized by being in a difficult medical communication situation.
It is difficult for most people to distance themselves emotionally from someone they love. When interpreting, a family member can get emotionally distressed by worrisome medical news, leading to them not being able to interpret correctly or continue interpreting at all. Some information might simply be too difficult to process while simultaneously having the responsibility of interpreting. Trained professional interpreters are neutral and can do this job without any emotional bias or judgment.
Honesty (and building trust) with a patient is necessary for a healthcare professional to give the right diagnosis and treatment. The patient, or a family member interpreting, may be uncomfortable discussing certain personal medical conditions. Or it might just be inappropriate (personally or culturally). That would hinder the healthcare provider to give the patient the right care.
One important duty of healthcare providers is to keep a patient's personal health information private unless there is consent from the patient to share this information. Using relatives and family members to interpret is breaching the confidentiality of the patient.
Family members might have their own emotional reasons to change a doctor’s message. For example, they might struggle with interpreting a difficult diagnosis, not wanting to burden the patient. Or they might want to control how the patient and the rest of the family are informed. For personal or cultural reasons.