。
We aren't always home on Sundays.
Literal
We [topic-は] Sunday [on-に] always house [in-に] exist [quotative-とは] not-limited.
The pattern [clause]+とは限らない is the standard 'not necessarily, not always the case' construction — literally '[it] is not limited to [clause].' Used to qualify generalizations and allow exceptions. 日曜日に always家にいる ('always home on Sundays') is the claim that gets qualified.