Is the mother of the stepfather's wife considered forbidden to the stepfather's sons?

The father's wife is forbidden to his sons and their sons, due to the Almighty's saying: "And do not marry those women whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah], and a most evil way." As for the mother of the father's wife, she is not forbidden to his sons, as there is nothing that necessitates prohibition. The Almighty has said: "And lawful for you are [all others] beyond these." Accordingly, she is considered a stranger to them, and hijab is obligatory for her, and it is not permissible to be in seclusion with her or to travel with them.