Day by Day by greywing- A fic about Izumi and Sig's relationship from Sig's POV.
One by greywing- Another fic that offers insight into the Curtis' relationship, this one from Izumi's POV.
Cookbook by __kirisaite- A drabble about the Curtis' relationship. Very short.
Knife by kaltia- A very amusing piece about how Izumi and Sig may have met.
Terms of Endearment by t3h_toby_chan- A fic set after the series about Sig and how he feels about Wrath, but some things about Izumi in it as well (as you can't really write about him without her).
Nicotine by Harukami- A dark fic about Izumi in the hospital following the transmutation.
In-Laws by Harukami- This is actually a Roy x Ed fic, but it's about Izumi and Sig coming by to see how the pair lives. Hilarity ensues.
Fragility by aguynamedgoo- Last one by yours truly, about the sexual aspects of the Curtis' relationship following the transmutation.
Got any more to add? Send'em my way!