Sasha was a female vampire and founder of the Denali Coven. She was only mentioned in Breaking Dawn. She was killed sometime before Carlisle met the Denali Coven after the Volturi discovered her creation of Vasilii, an immortal child.
Sasha was the creator of Tanya, Kate and Irina. They thought of her as a mother and are still upset about her execution. She was the one that created Vasilii, an immortal child. She kept his existence from Tanya and her sisters, to save them from the fate of being executed by the Volturi, which indicates that she expected it to happen.
Due to the danger of the immortal children revealing the existence of vampires, the Volturi got involved and executed Sasha by means of burning, with Vasilii in her arms. Tanya and her sisters witnessed their mother's death but were not destroyed afterwards. Their mother's death instilled a healthy respect for the laws of the vampire world in the sisters.
Added by LuckyTimothy