Are First Husband's Sons Mahram to Second Husband's Daughters?
Question

I got married to a man who was already married and who had sons. I had a daughter from him, then he divorced me. I married another man, who has died, and I have sons and daughters from him. Are the sons of the first husband who divorced me Mahrams to me and my daughters from the second husband? Is it permissible for me and my daughters to uncover our faces in their presence?
Praise be to Allah.Why First Husband’s Sons Are Mahram to Their Father’s Former Wife
The sons of your first husband are Mahrams to you , because you were the wife of their father and you are not permissible for them to marry, because Allah says (interpretation of the meaning): {And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.} [An-Nisa’ 4:22]
Mahram Relationship between First Husband’s Sons and His Daughter
Similarly, the sons of your first husband are Mahrams for your daughter from the first husband, because she is their sister through their father.
Are Daughters from Second Marriage Mahram to First Husband’s Sons?
With regard to your daughters from the second husband, there is no Mahram relationship between them and these sons, because there are no ties through blood, breastfeeding or marriage which would create that Mahram relationship.
Based on that, 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.
Permissibility of Marriage between First Husband’s Sons and Second Husband’s Daughters
Al-Kasani said in Bada’i` As-Sana’i` (4/4):
It is permissible for a man to marry a sister of his half-brother through his father. What that means is if his father’s wife gave birth to a son, and she had a daughter from another husband, that daughter is the sister of his half-brother through his father, but it is permissible for him to marry her. End quote.
This is the situation mentioned in the question. It is permissible for the sons of your first husband to marry your daughter from your second husband, who is the sister of their sister through the father.
For in-depth explanations, see the answers listed here:
Who Is a Woman’s Mahram
Etiquette of Talking to Non-Mahram Women
The Maternal and Paternal Uncles of the Husband Are Not Mahram to the Wife
Sitting with Husband’s Relatives and Shaking Hands with Them
Is the Father of One’s Ex-Husband a Mahram
Is My Husband’s Stepfather My Mahram?
And Allah knows best.