It's not necessarily the case that both parents are in good health.
Literal
Parents both healthy [not-necessarily-とは限らない].
~とは限らない means 'it's not necessarily the case that X' — a softer negation than わけではない. 健在 (alive and well / in good health) is a formal term. The sentence could be a general observation or a gentle way of saying one parent is unwell.