YOU SAID:

she was wearing a red sweater while she was reading a book

INTO JAPANESE

彼女は本を読んでいる間赤いセーターを着ていました

BACK INTO ENGLISH

She wore a red sweater while reading a book

INTO JAPANESE

彼女は本を読みながら赤いセーターを着ていた

BACK INTO ENGLISH

She wore a red sweater while reading a book