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It's not that both parents are alive.
Literal
Parents both alive [it's-not-the-case-that-わけではない].
~わけではない is a partial negation — 'it's not the case that both are alive,' implying at least one has passed. The total negation 両親とも生きていない ('neither parent is alive') would be much harsher; わけではない leaves room for nuance and softens what is still emotionally weighty content.