。
Mother isn't always at home.
Literal
Mother [topic-は] always home [at-に] is [not-limited-to-とは限りません].
とは限らない (here in polite とは限りません) is one of Japanese's nuanced denials, paired with the equally subtle わけではない. Both can deny a claim like 'mother is always at home,' but the feel differs — わけではない pushes back on an inference, while とは限らない denies a universal claim by noting that exceptions exist. The と is quotative-like (framing the preceding claim as a categorical statement), は marks it contrastively, 限る is 'to limit,' and 限らない is 'is not limited (to that).'