。
It's not as though mother is always at home.
Literal
Mother [topic-は] always home [at-に] is [not-exactly-わけではない].
わけではない ('it's not the case that') is one of Japanese's nuance-rich negatives — it denies not a single fact but a reasonable inference. The sentence isn't saying 'mother is never home'; it's saying 'don't conclude from [something] that mother is home all the time.' Pair with んじゃない (weaker) and ものではない (stronger prohibition).