2 Neil was studying when his sister was listening to music.

В данном предложении нарушено видо-временное соответствие глаголов. Действие «was studying» и действие «was listening» происходили одновременно в прошлом.

Исправленный вариант:

Neil was studying while his sister was listening to music.

Ответ: Neil was studying while his sister was listening to music.