Translation:She has four older sisters.
To add to joaquintoral's anwer, the character 个 (ge) is also known as a "measure word" or "classifier". Each noun has a measure word. "Ge" is the most common one. 条 (tiao) is used with nouns that refer to long, narrow things like towels, roads, snakes etc., 本 (ben) with things that are bound like books and magazines. It takes time to figure this all out.
Apparently if you simply want to use generic sisters or brothers you can use:
brothers = 兄弟
sisters = 姐妹
brothers & sisters = 兄弟姐妹
This is a problem across Duolingo Chinese: The audio version of this exercise doesn't allow for 他 OR 她 as correct answers. When it's written, the answer is clearly 她, but when you're asked to write the setence only from hearing it....then Duolingo needs to accept either 她 or 他 since it could be either one.
This is because you wrote 「他」, which is for "he", instead of 「她」, which is for "she". Note that the first radicals are different. The word for "she" has the girl radical, 「女」(nü3), and the word for "he" has the person radical, 「亻」(I myself am not sure how to read this, or if there is any pronunciation to it at all).
This policy was/is only used in the Peoples' Republic of China. There are many Chinese people living outside of it. Also, the one- or two-child restriction does exclude many Chinese minorities in areas of lower population densitiy. And there are also some people who are rich enough to not be bothered by the sanctions.
GT: In Chinese tradition, the personal pronoun "he" is a man and a woman, both male and female. The history of the personal pronoun "she" as a female "he" is very short, but it is in nearly half a century. Ever since, "she" can only refer to women, and "he" can refer to men and women.
No way to do that. You must say she have 2 older sister and 2 younger sister. Some of another East Asian languages have distinguish between older/ younger sister/brother, even the uncle who older/younger than father have different name, grandmother, grandfather who is father/mother of father, mother has different name. You must understand that East Asian countries (China, Japan, Vietnam, Korea...) have a high hiercharchy distinguishly social structure and their language reflect that. Dont apply "english" thinking when learning Chinese and another East Asian languages.
Time references in Chinese are mostly made by context.
她有四个姐姐。= She has four older sisters. = She had four older sisters.
本来她有四个姐姐，但是去年她们都死了。= Originally, she had four older sisters, but last year they all died.
她有四个姐姐，她们都上大学。= She has four elder sisters, they all are at the university.