Is the wife's sister considered a mahram?

The wife's sister is a stranger to her sister's husband; it is not permissible for him to look at her, be in seclusion with her, or shake hands with her. The prohibition mentioned in the Quran is the prohibition of marrying two sisters concurrently, not an absolute prohibition of marrying the wife's sister. She is not permanently forbidden to him. Sheikh Ibn Uthaymeen was asked about this and replied that understanding the wife's sister as a temporary mahram is incorrect. Mahrams are those permanently forbidden due to kinship or a permissible cause, and Allah did not mention the wife's sister among the mahrams.