The Koishikawa-
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Rikka with the family tree on Marmalade Boy Little.

Matsuura family tree is very complicated.

In the first episode of Marmalade Boy, Jin and Rumi divorced, swapped partners with the Matsuuras, and then remarried. Each couple had a child from their previous marriages. Jin and Rumi had Miki while Chiyako and Youji had Yuu. Rumi and Chiyako were both pregnant. Rumi had Rikka while Chiyako had Saku (also known as Hajime). Ever since the two children were born, who were also the main protagonists of Marmalade Boy Little, it further complicated the family tree.

Hajime and Rikka weren't blood siblings just like how Yuu and Miki weren't related by blood.