What is the Fatwa if I married my former step-mother, my former sister in-law?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is strictly forbidden for a Muslim to marry the wife of his father, since she is among the women with whom marriage is forbidden forever. Allah Says (what means): {And marry not women whom your fathers married, except what has already passed; indeed, it was shameful and most hateful, and an evil way.} [Quran 4: 22]

All the Muslim scholars have agreed upon the above ruling.

Marrying the sister of one's wife is permissible only after divorcing that wife by irrevocable divorce or after her death. There is no disagreement in this ruling. What is forbidden is to have two sisters as wives at the same time.

Allah knows best.