这里的since是连接词
conj.（连接词）
During the period subsequent to the time when:
在…以后的一段时间里：
He hasn't been home since he graduated.
他毕业后没回过家
Continuously from the time when:
从…时起一直：
They have been friends ever since they were in grade school.
他们从小学起就一直是好朋友
three years ago 是时间状语
妹妹一般是younger sister
姐姐 elder sister
只说sister 表示姐妹 既可以是姐姐也可以是妹妹