All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If this marriage (as you mentioned) had taken place without the consent of the guardian of this girl, then it is void according to the majority of the scholars as the Prophet said: "Any woman who gets married without the consent of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid." [Abu Daawood] Therefore, this marriage has to be nullified. However, since the guardian of this girl later accepted your brother's marriage to her, then the marriage contract can be renewed with the presence of her guardian and two witnesses after making sure that she is not pregnant by her observing one menstruation or giving birth to the child if pregnant. If there are any children from this marriage, they should be traced back to their father because he married her believing that it was a correct marriage (according to the scholars who are of the view that marriage without the consent of the guardian is valid).
The only thing that your brother and this girl should do now is to repent to Allaah for having established a forbidden relationship. Moreover, they have to conceal their sin, but there is no expiation for them or any religious worldly corporal punishment. As regards the punishment of the Hereafter, then we hope that Allaah will forgive them since they repented.
Allaah Knows best.