How to help my 28 year-old sister who believes in fairies and thinks she can shapeshift?

Your an adult and she is an adult. If you love your sister,you need to sit her down and start the conversations without the fairies. You need to be gentle but firm because she needs help. let her know that this isn't normal behavior and that she needs to see a doctor. Maybe she doesn't like the world and trying to escape it through her imagination. Has something happened to her in the past that triggered her to only daydream? You need to take her to the psychologist and just let her know she needs help.