Are the children of the first husband, who divorced the questioner, considered mahram to her and to her daughters from her second husband, and is it permissible for them to uncover their faces in front of them?

The sons of your first husband are maharim to you, as you are their father's wife. They are also maharim to your daughter from your first husband because she is their sister from their father. As for your daughters from your second husband, there is no mahramiyyah between them and the sons of your first husband, due to the absence of any relation that would necessitate it, such as lineage, suckling, or marriage affinity. Therefore, it is not permissible for your daughters to uncover in front of the sons of your first husband, and marriage between them is permissible.