What are these crests, anyway?

[Jey the Count]

Indeed, what are they? It semms like they're linked woth dragonblood (like in FE4). Both Sothis and Seiros are certainly devine dragons (just look at their hair! - the same as Naga, Nagi, Tiki, Nowi, Nah, Duma and Mila). What is more, the crest of Seiros is eerily similiar to the mark of the exalt (that unsrprisingly gives the ability to use the Falchion).

So: Did Sothis blessed Nemiesis and others with her blood (and maybe other dragons as well) to create these crests? What do you think? And how is the crest of flames/fire emblem connected to all this?

[Etrurian emperor]

The Golden Deer route has the answer. At the end they delve into the lore regarding crests. 

[Dr. Tarrasque]

As the above mentioned, you can find out through the Golden Deer route. If spoilers do not deter you from playing the game or you do not mind spoilers, you can see the scene where it's explained here:

 

SPOILERS that explains the Relics, Seiros, Crests, Final Boss of Golden Deer and the King of Liberation

Edited September 22, 2019 by Dr. Tarrasque

[CyberNinja]

17 hours ago, Hare'da-irijon said:

Indeed, what are they? It semms like they're linked woth dragonblood (like in FE4). Both Sothis and Seiros are certainly devine dragons (just look at their hair! - the same as Naga, Nagi, Tiki, Nowi, Nah, Duma and Mila). What is more, the crest of Seiros is eerily similiar to the mark of the exalt (that unsrprisingly gives the ability to use the Falchion).

So: Did Sothis blessed Nemiesis and others with her blood (and maybe other dragons as well) to create these crests? What do you think? And how is the crest of flames/fire emblem connected to all this?

They're simply the representation of having inherited the dragon blood that their ancestors stole. Major implies first generation or the one simply got lucky to get thicker blood. This is entirely different from crest stone which are the heart of dragons and those who utilize their power without a crest become monsters.