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It never crossed her mind that her son would commit suicide.
Literal
She [topic-は] son [subject-が] commits-suicide [quotative-とは] in-dreams-too did-not-think.
夢にも思わない ('never even dreamed, never crossed one's mind') is a fixed expression — 夢にも ('even in a dream') + 思わない ('not think'). The ~とは + 夢にも思わない frame creates 'never dreamed that X.' とは is the quotative と + topic-marker は, emphasizing the unexpected content of the 'dreaming.' Past tense 思わなかった narrates the past mental state. Often used to express shock at a tragic or surprising turn — perfectly fitting the topic of suicide.