。
Yoko's aunt is too weak to work.
Literal
Yoko [possessive-の] aunt [topic-は] body [subject-が] weak-too-much work-cannot.
A double-subject construction: 叔母さん is the topic, 体 is the subject of 弱い. 弱すぎて is 弱い + ~過ぎる + te-form — 'too weak, and therefore...' The causal te-form chains 'too weak' with 'can't work.' 働けない is potential negative of 働く.