My uncle sang a few songs,my aunt ----- the piano to accompa

很明显是独立主格,有逗号,没有连词,两个主语不一致,是with +名词+现在分词的独立主格模式（with省略了）
就是这个状语从句的变形：
when My uncle sang a few songs,my aunt was palying the piano to accompany him.