The mother died and her estate passed on to the girls when their grandparents died. The mother's family did not want his new children to get their grandchildren's inheritance so the court made the father legal guardian over the money or estate until they became of age. Maybe there wasnt a will when the mother died and she did not have any property. But the grandparents did have an estate and when they died their daughter's part was protected for their grandchildren by making the father guardian of their estate. They must have trusted him very much because they could have had someone else appointed administrator of their daughter's estate or granddaughters estates..