。
I couldn't attend the meeting because my mother was sick.
Literal
Mother [subject-が] sick [because-なので] I [topic-は] that meeting [at-に] couldn't-attend [explanatory-のです].
なので is the な-adjective/noun form of ~ので: 病気 (noun) + な + ので. 出席する ('attend') takes に for the thing attended. The trailing のです frames this as an explanation the speaker is offering — probably to justify an absence. A very practical pattern for excusing oneself or explaining circumstances.