、。
When mother isn't home, she looks after her little sister.
Literal
Mother [subject-が] doesn't-exist [when-とき], she [topic-は] little-sister [gen-の] care [obj-を] sees.
~ないとき ('when X isn't / when X doesn't') is the negative version of the temporal ~とき. Notice that 'when' in Japanese translates either to ~とき (any kind of time clause) or ~たら (conditional-flavored); they overlap heavily. 妹 ('younger sister') is specifically 'younger' — Japanese distinguishes older and younger siblings lexically.