if she is bored and you don't want her to disturb her sister,you should by all means give her some interesting books to read instead of asking her to write. Never force children under 4 years old to practice writing, they are not ready. you must stop that or else she will just hate it and never want to do that again. you may also buy her some line drawing exercise or colouring books. but if you really mean to let her learn how to write, then I have noting to say