Prohibition of a woman travelling without a mahram, and conditions of a mahram


Assalam O Alikium My mother is planning to go to ummrah inshallah. But she needs a mairram and her husband nor her brothers are able to go. We have asked an alm he said she can go with her brother in-law which is also her first cousin. As long as his wife is there which she will be cause she’s going too. Is this permissable in Islam because I still have my doubts. Jazak allah ho karun.