Unfortunately, your brother is not required to cooperate. He can cease the petition process at any time. A request to expedite could be filed but without your brother's cooperation, it will be impossible to bring your mother here. The petitioner must submit an affidavit of support even if a joint sponsor is available. His refusal to sign an affidavit of support will keep her from being able to immigrate. You can read more about family immigration at http://myattorneyusa.com/family-immigration.
Answered on Jun 16th, 2016 at 2:23 PM