I recently came upon Mahrams in Islam. I know that some of the Mahrams are clearly mentioned in the Quran as Allah says in Surat Al-Noor(24:31):
And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed.
I also found that the brother of your husband is not your mahram. As it's mentioned in a Hadith:
‘Uqba b. Amir reported Allah’s Messenger (may peace be upon him) as saying: “Beware of getting, into the houses and meeting women (in seclusion).” A person from the Ansar said: “Allah’s Messenger, what about the husband’s brother?” Whereupon he said: “The husband’s brother is like death.” [Muslim 26/5400]
However, I couldnt find any authentic reference neither in the Quran nor in Ahadeeth that talks about brother in law from the sister side(sister's husband). So is a sister's husband mahram in Islam? I am looking for answers with authentic references.