I can help you. Click on your profile picture.
Then edit my profile.
Finally, tap choose file and upload a profile picture. NO PERSONAL INFO LIKE FACES PLZ.
But lol, when i joined KidzNet my profile pic was my face, then KS reported it and texted me that faces were against the rules, I didn't know that tho, XD.
Also, if your sister got you an account on another device, you'll have to log in on yours. Do you remember what your password was? Maybe ask ur sister.