You can't "will" children. She could have a standby guardianship put in place but a Judge will determine who the children are placed with...In this case Dad/s would have the only real standing...'cause he/they IS/are the FATHER/s. Her hubby has ZERO standing to obtain guardianship...and really neither do you or any other family members.
I am going to disagree with that slightly.
The children's fathers would be first in line. If the fathers are legally unfit, or unwilling to take the children, or willing to allow the children to live somewhere else, then grandparents would be second in line, aunts and uncles third, and stepparents at the very bottom of the barrel, assuming that there was no one else available to take the children or assuming that all other parties ahead of the stepparents in line, would be willing to allow the children to live with the stepparent.
In other words, EVERYONE would have to agree to the children living with the stepparent, or the children would have to have no other living family.
I would hate for someone to think that if their child had no other living relatives, that a stepparent would not be able to get custody/guardianship.