The older set:
Leif - Level 10 Runethane - In charge of Xanadu's magic sector
Freja - Level 11 Greenbond - In charge of Xanadu's religion sector
The younger set:
Anika - Level 8 Ritual Warrior - In charge of Xanadu's trade sector
Halden - Level 7 Unfettered - In charge of Xanadu's entertainment sector
Leif and Freja are exceptionally magically inclined. Sigrid's decided that this could be due to the fact that she spent nearly her entire pregnancy in Xanadu (and with the Heart of the Sea). Whatever the reason, Leif and Freya are the most in tune with the Heart of the Sea (though neither of them has mastered it yet). They've basically been competing to master the Heart for most of their lives. Whoever controls the Heart will be the next ruler. Leif is the oldest, so he's the presumptive heir. This makes Freja a little annoyed (she's only minutes younger), so she works all the harder at controlling the Heart.
Anika and Halden don't really care about the Heart of the Sea so much. They might care if they could pull off some elaborate prank with it, but it's really tough to manipulate, so they lost interest. The younger twins certainly inherited their parent's troublemaking streak. Halden's in trouble with his mother nearly all the time. Anika doesn't get yelled at quite so much. Whether this is because she doesn't cause as much trouble or because she's better at not getting caught is not entirely clear. Lately, Halden's been spending a lot of time with Captain Blank. The younger twins are also both excellent swordsmen with light weapons.