夫が亡くなると彼女には赤ん坊が何より大切になった。

。

When her husband died, the baby became more precious to her than anything.

Literal

Husband [subject-が] passed-away [conditional-と] her [to-に] [topic-は] baby [subject-が] anything [more than-より] precious became.

何より大切になった = 'became more precious than anything.' ~と here is the conditional expressing an inevitable consequence following an event.