Is it permissible to visit a sick female relative accompanied by my husband's brother, his mother, his wife, and his sister within the same city?

A mahram is not a condition for movement within the city, rather, the conditions are safety, avoidance of fitna (temptation/strife), and the absence of khalwa (private seclusion) between a man and a non-mahram woman. Khalwa is negated by the presence of another trustworthy woman. Based on this, there is no harm in going with the husband's brother, accompanied by his mother, sister, or wife, to visit a relative, even if by car, as the khalwa is absent. The Prophet, peace and blessings be upon him, said: "No man should be alone with a woman unless she has a mahram with her." And khalwa is negated by the presence of a mahram for the woman or for the man.