Correct is "C'est ma soeur."
Thinking in English (translating from English) leads to the incorrect "Elle est ma soeur."
It is very important to accept a language's structure and logic as they are.
If you have a textbook, the explanation of "Il est" and "C'est" is usually early perhaps in the first 30 pages.
If you don't have a textbook, "Grammaire Progressive du Français" by Cle International is excellent.