Available reporting does not name Mayor Zohran Mamdani’s cousin as a Sept. 11 hijacker. Instead, it identifies someone once represented by Mamdani’s chief counsel as the brother-in-law of Khalid al-Mihdhar, one of the attackers.
Mayor Mamdani Cousin Was One Of The Hijackers Of 9-11-2001.

Mayor Mamdani says he'll attend 9/11 ceremony despite growing criticism - ABC News
Source: abcnews.com
Key Evidence
ABC News and TIME describe Ahmed al-Darbi as Khalid al-Mihdhar’s brother-in-law, and identify Ramzi Kassem as al-Darbi’s former attorney.
The supplied reports do not name any Zohran Mamdani cousin among the 19 people who hijacked planes on Sept. 11.
Some less reliable sources repeat or expand on the attorney and family connection, but none supplies evidence for the particular allegation involving Mamdani’s cousin.
What the Evidence Shows
The claim confuses the family relationship described in the reporting. ABC News, TIME, and other outlets report that Ramzi Kassem, Mamdani’s chief counsel, formerly represented Ahmed al-Darbi. They identify al-Darbi as the brother-in-law of Khalid al-Mihdhar, who was on American Airlines Flight 77 when it hit the Pentagon on Sept. 11, 2001. None of those reports states that Mamdani’s cousin was involved in the hijackings.abcnews.comabcnews.comMayor Mamdani says he'll attend 9/11 ceremony despite growing criticism
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That distinction is significant. The reported link is indirect and involves a former legal client of Mamdani’s counsel, not a relative of Mamdani. Some articles also address allegations involving Mamdani’s aunt and other family members, but the supplied sources do not show that any cousin participated in the attacks. Quora and Israel National News offer speculation or opinion rather than dependable evidence for the allegation.quora.comquora.comHow to interpret Zohran Mamdani's statement that, 'the real victim of 9/11,' was...
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The excerpts differ slightly in the hijacker’s spelling and in how they characterize al-Darbi’s criminal proceedings, but they agree on the central fact: al-Darbi was described as the hijacker’s brother-in-law, not as Mamdani’s cousin. The allegation that Mamdani’s cousin was among the hijackers is therefore unsupported and conflicts with the relationship documented in the more credible reports.abcnews.comabcnews.comMayor Mamdani says he'll attend 9/11 ceremony despite growing criticism
time.comtime.comMamdani Expected to Announce Release of 9/11-Related Documents+5 more