Linda Chapman 
My Secret Unicorn: Flying High 

The third book featuring Lauren and her pony Twilight who changes into a unicorn when Lauren says the magic words. Lauren’s friend Tilly doesn’t want her father to re-marry. She doesn’t get on with her future step-sister and doesn’t know what to do. On the day of the wedding Tilly runs away and is found in Twilight’s stable. Twilight conjures a vision of the future and shows Tilly how miserable her family would be without her and she agrees to go back. Lauren and Twilight use magic to help Tilly return to the wedding in time. In return Tilly promises to drink a special potion which makes her forget everything that Twilight and Lauren have shown her and keeps Twilight’s secret safe.