I'm not going to tell you the word, I'm going to show you how to find it.

1. Click here to be taken to the Translator.
2.  In the little drop down box, change it to "English to Dragon"
3. Type "Sister" in the box.
4. Click the "Translate" button.
5. Click on the underlined word below the button.
6. You have your translation!

(Note, I tested this as I wrote the steps, it works.)