Она была гораздо моложе своей сестр

Она была гораздо моложе своей сестры и менее серьезна

Результаты (английский) 1:

She was much younger than his sister and less severe

Результаты (английский) 2:

She was much younger than her sister, and less severe

Результаты (английский) 3:

she was much younger than her sister, and less serious