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.
Both the 16-year-old and the 2-year-old have the same mom - OP’s mom.
Why “daughter” and “child” then? Not daughters, age 16 and 2, or daughter and son? If it were the reverse I’d assume the teen was nonbinary but it’s unusual in a toddler