My late husband asked me not to remarry. Is it a sin if I marry again?

It is not a sin; that request is not binding, because a widow has the right to remarry after her waiting period (iddah). Requests like this usually come from deep love, and deserve to be remembered as a sign of affection. But they cannot block something Islamic law permits. A will is binding in matters of property and lawful things, not to restrict the life of the one left behind. If you choose to stay single, that too is an honourable choice. If you want to remarry, make the decision calmly, with istikharah, and with your children's welfare in mind. Keep praying for your late husband, and live your life without guilt. See guide for widows.