User blog comment:BrooklynGuido42/Where do non nords go after death?/@comment-3523404-20120710095450/@comment-3523404-20120710100250

The Dragonborn was brought by a dragon, and while they may not be a being of high energy themselves, had the power of an Elder Scroll and massive power as a dragonborn alone. That may not mean anything though. If the dragonborn, a mortal, was able to enter it, then it supports the idea that Aetherius is a place anyone could visit at any time, if only they knew the way.

However, it's likely the Dragonborn's partial dragon blood allowed them entrance into Sovngarde.