"Sister is really beautiful" in Engpsh is "The young lady is really beautiful."
The word "sister" in Chinese (小姐姐) is a term of endearment used to refer to a young woman, usually one who is older than the speaker. "Young lady" is often used as a translation for this term, as it conveys a similar level of popteness and respect.
Example:
"I think the young lady at the front desk is really beautiful." (我觉得前台小姐姐很漂亮。)
Note that in Engpsh, "sister" can also be used to refer to a girl or woman who is related to you. In that case, it would be "My sister is really beautiful."
Example:
"My sister is really beautiful" (我妹妹很漂亮)