Is it permissible for the questioner's wife to appear before the sons of his sister who nursed the questioner's daughter?

Lactation establishes the same prohibitions as lineage, for the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is forbidden by lactation." However, this applies exclusively to the nursed child, not to their siblings, their brothers and sisters, or their ancestors. Based on this, your sister's children do not become mahram to your wife merely by her nursing your daughter, and she is not permitted to appear unveiled before them if they have reached puberty.