Can citizenship be revoked?

It is most likely "unilateral marriage fraud." That means that the fraud was committed completely on the part of the alien while your sister was married in good will. Yes, this is illegal. Could he have his citizenship revoked for this? Yes, because he obtained it through fraud. What is the likelihood of it actually happening? VERY SMALL. Your sister should write a letter to USCIS explaining everything. She should also call or write her congressman's office.

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Yes. If within two years of becoming a citizen the USCIS finds that the person acquired citizenship through misrepresentation, then administrative revocation can take place. If the time limit of two years has expired the government must file suit in federal court to have the person "de-naturalized".

*If a US citizen renounces his citizenship at a US Embassy overseas and then becomes a citizen of the foreign country, the US government will try to take away his US citizenship legally. Other countries will revoke citizenship if the person becomes a citizen of another country. This is hypocrisy since many countries claim that their people are still citizens even if they are born in another country,as long as one of the parents is a citizen by birth. Many people have dual nationalities and two passports which they use regularly. Jews born outside of Israel are an example where regardless of where they were born, they have the right to ask for and be granted citizenship in Israel without a lengthy wait.