I am August Abbott, formally trained x12 yrs private school:international protocols, etiquette. Please allow a few mins. to compose your response.
This is a very good question and it tells me a couple of important things about you: First, that you care about being polite and considerate; secondly, that you know in life it’s inappropriate to criticize anything (or anyone) without offering a solution or two at the same time.
I would expect that your sister already knows she’s got a problem with the cabinets and probably feels she’s got to adjust and just live with them the best she can since it’s a finished job. So saying something will only add to her feelings of frustration and possibly regret at hiring the installers she had and then paying them for a job ‘not’ well done.
If you were to say anything you might want to offer your help in attaining a solution. One day while you’re trying to reach her upper shelves, turn to her and say, ‘If you want to have these lowered just let me know and I’ll pay half the bill. It would be worth it just to be able to reach up here’ and keep your attitude light.
Meanwhile, buy her a nice step stool that matches her kitchen. This could be another way to broach the subject and make a kind offer to help her have this work re-done.
I have no doubt that you’ll handle this quite adeptly as you impress me with being a thoughtful and kind sister.