All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is well-known that it is forbidden to sell alcohol in one's work; the Prophet, sallallaahu ‘alayhi wa sallam, cursed the one who sells wine. As for your brother who cut off relations with you because you refused to work in his supermarket where he sells alcohol, then he bears the sin of cutting the ties of kinship and not you. It is impermissible for you to do what he is asking and work with him in selling alcohol even if he continued to cut off relations with you.
What you are permitted to do is to keep relations with him within the limits of what Allaah has made lawful; if he insists on cutting off relations with you, then he bears the sin for that and you bear none. In this case, the stern warning against severing the ties of kinship in the Hadeeth that reads, "The one who severs the ties of kinship will not enter Paradise," does not include you.
However, if you are in need and cannot provide for your basic needs such as your medication and the like, then you are allowed to work with him out of necessity as underlined in Fatwa 90691.
If you can work in the supermarket without having anything to do with personally selling alcohol or facilitating that, then there is no harm in working with him.
Allaah Knows best.