This takes place in the friendship is witchcraft universe. From left to right:  
Toxic Heart is the child of Celestia and Rainbow Dash, it was an accident.  
Mistress Shadow is the adoptive daughter of Luna and another mare. No one knows who, but Luna entrusted her child to Rarity, the element of generosity, because Rarity wad generous enough to give her time for the child.  
Demian is the element of acceptances and takes his place among the others when he is needed. Before this he refused to admit his skills.  
Last but not least, Scarlet Blaze is the element of magic. Though she is not a unicorn, she accessed a previously unknown magic through her ability to fly fast. Like Rainbow Dash’s sonic rainboom, when she reaches a certain vosity her body vibrations can allow her to teleport between the world of FIW and FIM.