What rights does she have if my daughter father passed away and her sister has power of attorney?

Unless there is a surviving spouse, a child is the next of kin. A sister with a power of attorney has no rights, a power of attorney becomes void at death.

A POA terminates at death. Your daughter has the equal rights to her father's probate estate as her siblings. If there is no representative appointed in his will, then your daughter may apply to be administrator of his estate if a probate administration is necessary.