Well, I'm conflicted as to how to advise. In my experience, the easier the sax part is, the more is expected of the piano accompaniment to "fill" in while the sax focuses on the melody. As the sax part increases in complexity, the piano accompaniment does not decrease but stays the same or increases in difficulty.
Just what frustrates your sister? If she's a beginner, she should play tunes commensurate with that level. What does her instructor say about her frustration? If she doesn't LIKE the tunes she's playing then find something that she DOES like and focus on that for you two while she has to go through the required mechanism of learning to play the piano.
Best advice I could get is go to sheetmusicplus.com, find some easy Broadway, Disney, or Pop tunes she likes and can play on piano. Buy the piano books and you adjust your play accordingly. You may be able to find corresponding sax music to play along with the given books mentioned above. Probably be very basic/simple/easy but not then that difficult for you to embellish and practice on some level of improv and fill-ins! Especially if you've already got the majority of the tune/melody in your head!
Good luck and let us know how it goes!