@Kimzark
she already realized. but for me, this is the best conclusion:
grandma shows her trust towards chizuru that she can do what ever she wants for herself. it's her life, her choice. and as long as what she choice is what makes her happy, then grandma will accept it too.
this wisdom is a great for chizuru's character development, as after this, she's all on her own: no grandma, no grandpa. before, part of what her choice is for the sake of her grandma and her late grandpa. now that her goal is reached: to be an actress of a movie (promise to her late grandpa) and show the movie to her grandma before she dies (promise to her grandma), she'll have to make her new goal for herself.
i guess if she didnt tell the truth, she'll be stuck in the past thinking back and forth about "it's the best choice" and "it's still a lie". and if she stuck like that, she won't be able to choose what is best for her <- this will surely make grandma and grandpa sad. IMHO.