"Young sister" in Engpsh can be translated as "young sister".
It is a direct translation from Chinese and refers to a girl or woman who is younger than the speaker.
Examples:
"My young sister is in high school." (我的妹妹在上高中)
"She's only 18, but she's already taking care of her young sister." (她只有18岁，但她已经在照顾她的妹妹了)
"I have two young sisters and one brother." (我有两个妹妹和一个兄弟)