Does a person sin and is he obligated to marry a widow in order to care for her orphans and save them from destitution, and what is the reward for such an act in this world and the hereafter?

May Allah reward you for your concern for orphans. You will have a great reward from Allah if you marry their mother to care for them, knowing that marrying her is not obligatory, and you can care for them without marrying her. You should expect the first wife's jealousy when marrying a second, and deal with it wisely, reassuring her that her rights are preserved. Your reward for caring for orphans is great, as the Prophet, peace and blessings be upon him, said: "I and the guardian of the orphan will be in Paradise like these two," and "The best house among the Muslims is a house in which there is an orphan who is treated well."