Sid's family are a group of ground sloth who are related to Sid. However, Sid was left behind when the migration came. Although they have made no appearance in Ice Age, Sid called out the names of them: * Zak * Marshall * Bertie * Uncle Fungus Zak and Bertie never appeared, but considering that Sid, in the fourth movie, defines Milton, Eunice, Marshall, Fungus and Granny as his whole family, not mentioning or asking for Zak and Bertie, most likely these two were simply friends that have lived for a while with Sid's family.