So far Sofia the First has featured many Disney princesses and associated characters1. Does this mean that all Disney princesses are part of the same world, is there some sort of magic portal between Enchancia and the worlds of the other princesses, or is this some sort of altered universe that happens to contain the other princesses?
The princesses are summoned magically through time & space by the Amulet of Avalor:
The wearer's good deeds bestow magical abilities; whereas bad deeds cause the wearer to be cursed. Also, according to Cinderella, the Amulet connects all of the princesses that ever existed.
Princess Summoning: When the one wearing the Amulet really needs help, the Amulet summons Disney Princesses to help them out.
It can summon Princesses from the past, according to Cinderella, "all princesses that ever lived." Interestingly, it seems to be implied by Tiana's appearance that princesses from the future can also be summoned.
This is explained in the very first episode by Cinderella.