Any document showing he is your uncle like rashan card etc.

Regards,

Plz check it is a free answer

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There is no provision in law to "sponsor for F1 visa".

However, someone may offer to meet expenses of your education and may need to show that his financial worth to the concerned school. If the school is satisfied with financial document, they may make a mention of the same in I-20. No separate document , contrary to what has been mentioned in I-20, may be required to be shown to the consulate at the time of visa interview.