Is it permissible for my sister to uncover before her father-in-law's sons, and for my sister's daughters to uncover before their paternal cousins, knowing that my sister's father-in-law's wife breastfed three of her children (one daughter and two sons), and that my sister did not breastfeed any of her father-in-law's sons, and that my sister's husband and her father-in-law are paternal cousins?

It is not permissible for the sister to uncover her hijab (definition) in front of her husband's paternal cousins, because she did not suckle from their mother. Nor is it permissible for the sister's daughters to uncover their hijab in front of their father's paternal cousins, except for the daughter who suckled from their mother, as she is their suckling sister. As for the others, they have no relation to the suckling that occurred. The sister's children who suckled from that woman are considered mahrams to her daughters, because they are their suckling sisters.