Well, okay then. SPOILERS AHEAD YARR.
Assuming you've read all the english stuff, you'll know that Yori (brother) moved away from home to go to another school, so his parents wouldn't find out he's sleeping with his sister. Well, his old girlfriend gets pics of Yori and Iku kissing, and eventually manages to get Yori kicked out of his new school. (He had snuck Iku into his dorm room for some hot sibling passion, but through an unfortunate set of events Yori's old girlfriend gets some guys to almost rape Iku.)
So Yori moves back home, and eventually Mom figures out they're sleeping together. The two move in with Yano, their mutual friend, but Yori and Iku's mom gets sick, so they go back. Turns out they aren't fully twins: The mom had sex with two men, her husband and her friend, and they both got her pregnant simultaneously. So Iku and Yori are only half-siblings. Nonetheless, Yori decides to leave the house, and moves away. Iku cries lots. 10 years later, she's on a business trip in England when she bumps into someone in a library. That someone is Yori. They embrace, and the series ends. /spoiler]