My sister is impatient, so she often forgets things.
Literal
Younger-sister [topic-は] impatient so often forgotten-things [object-を] do.
せっかち ('impatient, hasty') is a na-adjective describing someone who rushes through things. だから ('so/therefore') marks the causal link. 忘れ物をする ('to forget things') is a standard collocation — 忘れ物 literally means 'forgotten item.'