I think you need to understand that regardless your child will feel grief: pain, anger, upset, sadness over the adoption and her birth parents. She might not even tell you - sometimes kids keep it inside. I do think that telling her will bring an uprush of emotions for her, happy and sad. But not telling her wouldn't make the emotions not exist - wouldn't make them bubble up.
Personally, I think you should tell her.
Just say simply, I sent a letter to your grandmother (name) and aunt (name). They write back and are so happy. Would you like to draw them a picture to send them. And then deal with any and all questions that come forth.
She is in your family now - she will not move.
They are still a part of her family and love her greatly.
It is happy to hear from them and sad to have had to leave them.
People get sad at times like these. And happy.
Big hugs for her.
That is my honest opinion as a life thriver.
Maia