Is concealing the marriage of one's sister after the death of her husband considered deceit and fraud, given that the family of her former husband believes she has not remarried and continues to cover her expenses and those of her daughters, and does sin fall upon those who know about this and conceal it?

The sister is not obligated to inform her former husband's family of her marriage. It is permissible for her to benefit from any money they give her as a donation, unless she knows or strongly believes that they spent on her because she had no provider, and that if they knew [about her marriage], they would withhold [the money]. In that case, it is not permissible for her to take it. Similarly, the money given to the children may be taken unless proven otherwise. Marriage has conditions, such as a guardian (wali) and witnesses. Announcing it is recommended but not obligatory, though it is better to announce it to avoid corruption.