To answer your specific question, you don't have anything to do with your mother's petition, unless you had to be a co sponsor. If you were a US citizen (which you didn't specify), it is a longer process for you to Petition for him, as opposed to your mother. Perhaps there is a better option but you would need a formal consultation with an attorney for tailored advice.
You cannot do anything to help your brother at this time. He must wait for an immigrant visa to be available for him and his family.
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He just needs to keep waiting until his priority date becomes current. Since he is married, he falls under a third preference category, and just has to wait for his visa to become available. As a US citizen, you may also petition for him, but this petition would fall under a fourth preference category with waiting time of approximately 10 years or so. In either case, go ahead and file a petition for him as well. He has nothing to lose, and in case something happens, he will have another petition filed on his behalf.
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You must mean that your mother submitted a visa petition for your brother's immigration--
green card, not citizenship. The quota for a married son of a US citizen (FP-3) is 10 years, so it will likely take another five years. Check the monthly visa bulletin for updates. The wait for siblings of US citizens (FP-4) it is even longer. But your brother has a bigger problem: Because he overstayed and is likely out of status, he may not adjust his status in the U.S. , and by departing to consular process, he will become inadmissible for 10 years. He will need a waiver of inadmissibility based on a showing of extreme hardship to his US citizens parent. It is a complex process and you should not try to tackle it yourself.
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