Is the wife's son, whom the husband is raising, considered a mahram to the husband's sister, or is he a non-mahram to her?

Women forbidden for marriage are either so by consanguinity (mothers, daughters, sisters, paternal aunts, maternal aunts, nieces, grandnieces), or by affinity (wife's mother, wife's daughter—provided the marriage with the mother has been consummated—son's wife, father's wife), or by suckling, for the Prophet, peace and blessings be upon him, said: "Suckling prohibits what consanguinity prohibits." A man is a mahram to these women, and a husband is a mahram to his wife. The son of a brother's wife is not a mahram, and a woman must observe hijab in front of him.