Could my daughter, who is a lawyer, defend my son’s girlfriend, who killed my cousin’s family in a DUI accident? - Sopuli

Hypothetically, my daughter is a defense attorney. My son is dating a woman who, while driving under the influence, kills my cousin’s 16-year-old daughter and her 2-year-old child. My son asks his sister to defend his girlfriend—would that be legally allowed? For context, my cousin’s father is my mother’s brother.