Is the father's wife an aunt?

My father is married to a woman whom he divorced after she bore him a daughter who is older than me. Is my sister's mother my aunt?
She is not your aunt. In reality, your aunt is your father's sister, but she is considered a mahram (a relative one may not marry) to you even if your father divorced her.
And all praise is due to Allah, Lord of the worlds. Glory be to You, O our Lord, and praise be to You. I bear witness that there is no deity but You. I seek Your forgiveness and turn to You in repentance.