Yes. A deed is legally accepted if it is recorded.
If both you and your mom sign it and it is notarized that fulfills two of the three parts necessary to be legally binding: signed, sealed and delivered. Delivery can be accomplished by handing to deed to the new joint tenant, by handing to an agent for the new joint tenant, or by recording the deed. (Of course, this is only valid if mom is competent and not under undue influence and isn't committing a fraudulent transfer to defraud her creditors.)
If the new joint tenant has not personally accepted the benefit of the joint tenancy, however, the new joint tenant can disclaim the gift of real estate and reject the gift. This could happen in this case if it was done in the proper manner (established by state law) in the proper time frame. If the new joint tenant disclaimed the gift, the new joint tenant would be treated as predeceased you, and you would become the sole owner of the property.
Someone might disclaim a gift, for example, because it would just go to their creditors or would disqualify them for public benefits that are means tested or because the property might come with toxic waste liability.
Of course, while you have represented that you filled out the deed correctly, without seeing it and knowing which state's law applies, there is no way anyone over the Internet can confirm that you did it correctly.
While it wouldn't invalidate the transfer if it was not done, there are also tax obligations associated with the gift of a joint tenancy interest. First, you and your mom have a duty to communicate her tax basis in the property to the new joint tenant (basically purchase price adjusted for capital improvements and depreciation taken for tax purposes). Second, if the value of the joint tenancy interest exceeds $15,000, your mom must file IRS Form 709 (a gift tax return) disclosing that the gift was made to your brother and establishing the value of the gift. Form 709 is due on April 15 of the calendar year following the calendar year in which the gift is made (subject to adjustments for certain holidays and Sundays).