。
Mother isn't always an early riser.
Literal
Mother [topic-は] always early-rising [not-necessarily-とは限らない].
~とは限らない is a fixed 'it's not necessarily the case that' construction. と is quotative-ish (framing the preceding claim), は marks it contrastively, 限る is 'to limit,' and 限らない means 'is not limited (to)' — i.e. not constrained to that single outcome. Pair with 必ずしも~ない for the stronger 'not invariably' flavor. Here, the claim 'mother is always an early riser' is being denied as a categorical.