Legal Question in Immigration Law in Missouri

One does not automatically become a citizen if they are not born in the United States, even to a U.S. citizen parent(s). Her children would not automatically become U.S. citizens either. Different determining factors include if the parents were married, if the father was a U.S. citizen, if any of them ever lived in the U.S., if the child ever lived with the U.S. citizen parent, etc.

I recommend that your sister contact the U.S. Consulate in the country that she is living in for more information, or contact a U.S. Immigration Attorney for a more detailed evaluation of her circumstances.