Barbara Pierce Bush endorsed Kamala Harris for president in an exclusive statement to PEOPLE on Tuesday. She campaigned for Harris with friends in Pennsylvania over the weekend
It's a commonly used term. The President's family is the First Family. All children are the First Son/First Daughter even if there are several siblings. "First Lady" isn't an official term; it's not mentioned in the Constitution as an actual position but traditionally it is a thing.
The first time I heard this term, I was so confused both as a young kid and as a child of immigrants (at that time, English was not my first language).
I couldn't understand why they were first and then when another president came in they were first again.