Under no circumstances can women perform Hajj without mahram. And brother-in-law is not mahram. So it is not permissible for a woman to go on Hajj with her brother-in-law even if she is with her sister. So the woman has to stay with her mahram. Because , in Hajj, every woman has to have her own mahram. Yes , it may be that one person is mahram of some , so he can go on Hajj with them. For example, a father may go on Hajj with some of his daughters. Since the father of each of the girls is mahram. So there is no problem. Hadith Sharif , Ibn Abbas. The Prophet (peace and blessings of Allaah be upon him) said : No woman will travel without her mahram, and no man will approach a woman without her mahram. Then a man said , O Messenger of Allah! I want to go to Jihad with such an army and my wife wants to go for Hajj. The Prophet (peace and blessings of Allaah be upon him) said , ' Go on Hajj with him. ' (Sahih Bukhari 1/146 , 1/250 ; Badayous Sanaye 2/299 ; Almuhitul Burhani 3/394 ; Addurrul Mukhtar 2/484 ; Mukhtasarut Tahabi 59 ; Khulasatul Fatawa 1/26 ; Jubdatul Manasik 32)