There is no Mahram relationship between your daughters from the second husband and the sons of your first husband, because there are no ties through blood, breastfeeding or marriage which would create that Mahram relationship. So, it is not permissible for them to uncover in front of the sons of your first husband, and it is permissible for them (the first husband’s sons and your daughters) to marry one another. However, they are Mahrams to you.