Is it permissible for the children, mother, or siblings to object to the marriage of their widowed sister who secretly married a non-Arab man who does not pray? And is this marriage considered legitimate according to Islamic law?

If a woman marries herself without a guardian (wali) and two just witnesses, her marriage is invalid, based on the Prophet's (peace be upon him) saying: "Any woman who marries without the permission of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid," and "There is no marriage except with a guardian and two just witnesses, and any marriage other than that is invalid." The marriage should be annulled by referring the matter to the Sharia (definition) courts. The children of this woman must understand that her action does not negate their duty to treat her kindly and with humility, as Allah Almighty says: (And your Lord has decreed that you worship none but Him, and to parents, do good...) and as the Prophet (peace be upon him) said when asked: "Who among people is most deserving of my good companionship?" He replied: "Your mother," three times, then said: "Your father."