hi, im a native japanese speaker.
when i was in my first grade of a elementery school, i was taught how to wirte hiragana, katakana and kanji in this order.(you know kanji is more difficult to learn than the others).
you said that your sister has already studied hiragana, so i think you should teach katakana to her, and then kanji.
and one more thing, you said that it is a little hard for her to remember.
then ill recomend some japanese mangas, you know?
of cource in english at first, i mean if she read it, she will understand some of the japanese cultures or something, anyway she will get something from it!
and the last thing to say, if she gets to the level to study kanji,
please let her watch these videos.
she will be studying and remember with fun:
hope this will help you.