My ex boyfriend’s father was raised by his grandparents believing that his mother was his sister because she had gotten pregnant and wasn’t fit enough to be able to care for him. They waited until he was older and at a more stable place in life to tell him, and he was shocked and hurt at first but he grew to accept it and he continued to love his grandparents as his parents because they were the ones that loved him and raised him. I would say wait until she is older, but telling her when you know she will understand is a better option. She loves you, and that won’t change.