。
She was cautious in everything she did.
Literal
She [topic-は] what [object-を] do [in-にも] cautious was.
何をするにも is a productive embedded-question pattern: 何 + を + する + にも = 'in (whatever) doing / in any action.' The にも turns the embedded clause into a universal scope — 'no matter what she did.' 慎重 ('cautious, careful, prudent') is a na-adjective applied to character or behavior. The であった form is the formal/written variant of the past copula だった, giving the sentence a more polished, descriptive register fit for biographical writing.