Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
No, it is not permissible to be a Mahram for her, as sister in-laws are not permanent Mahram.
If she has any other Mahram relatives (i.e. father, son etc,) to accompany her then it will be permissible. If however, she does not have any Mahram relatives to accompany her then it is not permissible for her to go for Hajj.
Even if, till her death no Mahram is available, she cannot go for Hajj, rather it becomes Fardh upon her to make Wasiyyat (will) of a Hajj-e-Badal. (Ahsanul Fataawa part 4 pg.532)
