Can sister go for umrah or Hajj with her sister and Brother in-law.
Her brother in-law is not a Mahram for her. Therefore she should not travel for Hajj or Umrah in this kind of situation. She should wait until her Maram such as her brother, father, uncle, grand father, nephew or husband can travel with her.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America