Is she considered a sinner, the one who poured hot water on ants, thus killing them, even though she was keen not to kill them?

It is not permissible to kill ants because the Prophet, peace and blessings be upon him, forbade the killing of four animals, including the ant. However, if ants attack a person in their belongings or dwelling, and their harm reaches a degree where it cannot be repelled except by killing them, then it is permissible to kill them, provided that they are not killed by fire or hot water. If the presence of ants was harmful and the water poured on them was not hot, then there is no sin upon her.