、。
Since mother was sick, I took care of her.
Literal
Mother [subject-が] sick was [because-ので], I [subject-が] mother [gen-の] care [obj-を] did.
Notice the double が here: mother in the subordinate clause (subject of the ので-clause), and 私 in the main clause. This is the default for subordinate + main clause structures — the topic-marker は prefers main clauses, and subordinate clauses take が for their local subjects. 世話をする is the standard 'take care of' noun-する compound, parallel to 面倒を見る.