To say sister in Thai you have to specify whether it’s a younger or older sister.

Younger sister

When it’s a younger sister you would say น้องสาว nóng săao.

Older Sister

To say older sister in Thai you would say พี่สาว pêe săao

Examples:

Do you have any sisters?
มีพี่สาวหรือน้องสาวหรือเปล่า (literally: do you have older sisters or younger sisters?)
mee pêe săao rĕu nóng săao rĕu bplào

How old apart are these sisters?
พี่สาวและน้องสาวคู่นี้อายุห่างกันเท่าไร
pêe săao láe nóng-săao-kôo-née aa-yú-hàang gan tâo rai

Summary

To summarize, older sister is พี่สาว pêe săao and younger sister is น้องสาว nóng săao.

พี่ pêe is actually a word you will see often that refers to someone older than you while น้อง nóng is used to refer to someone younger than you. For example, the words for brothers follow the same pattern.

สาว săao by itself means young woman.