Children are not property and cannot be disposed of within a will. While the mother can nominate a guardian for her minor child, it is the court that has the final say in who will have custody of the minor child. Who is nominated as the guardian is only one factor as the court considers what is in the best interest of the child.
Unless there was a court order terminating the father's parental rights, it is doubtful that the father gave up all his legal rights and responsibilities. The father may not have seen his child for a few years. However, if the mother dies, the father may become interested in his child again. Perhaps it's a change of heart. Perhaps it's greed (if there is a possibility of the mother leaving a sizable estate).
That you are the child's grandparent does not necessarily give you a better chance at getting custody of the child. For example, if you had not been involved with the child or had been abusing the child, you may not be a candidate for custody of the child.
Have you talked with your daughter to find out her reasons?
A will cannot "dispose" of a child, and grandparents have no presumptive right to custody.
In the event of a parent's death, the court has the power to appoint a guardian ad litem for the children and to determine the best placement for the children. It is unlikely that a short-term boyfriend would be the placement in the best legal interest of the children, but if the relationship has lasted a long time, and the children have a long-term, close relationship with the boyfriend, it is possible.
Grandparents are entitled to file motions for visitation and custody, and if the court deems such to be in the childrens best legal interest, the court may award such visitation and/or custody. If your relationship with your daughter and your grandchild is close, consider offering to pay for a joint consultation with an experienced family law attorney who can help your family develop an appropriate plan for the children.
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