。
She's been sick since last Wednesday.
Literal
She [topic-は] last-week [possessive-の] Wednesday [since-から] sick is.
から marks the starting point in time — 'since Wednesday.' The present tense 病気です indicates the illness is ongoing. 先週の水曜日 uses の to connect the relative time frame (last week) with the specific day (Wednesday) — a standard pattern for specifying dates within a broader period.