。
My sister saw it with her own eyes.
Literal
Younger-sister [topic-は] that [object-を] own-self [possessive-の] eyes [with-で] saw.
自分自身 ('oneself') doubles the reflexive for emphasis — 自分 alone means 'self,' adding 自身 intensifies it to 'one's very own.' の目で ('with one's eyes') uses で for the instrument of seeing.