Haruto is the brother of Satomi, who is Aiko and Kenji's mother. This makes Haruto Aiko and Kenji's uncle, and consequently, his children Daichi and Hina are their first cousins. In regards to Yuto, since he shares the same mother, Satomi, with Aiko and Kenji, he also has the same relationship with Haruto and Haruto's children. Therefore, Daichi and Hina are Yuto's first cousins as well.

Mariko is another child of Aiko, Kenji, and Yuto's maternal grandparents, along with Haruto and Satomi. This establishes that Mariko is the sister of Aiko, Kenji, and Yuto's mother. Hence, Mariko is their aunt.
Aiko is Kenji's sister, and they both have an elder brother, Yuto. Their maternal grandparents have three children: Haruto, Mariko, and Satomi. Satomi is Aiko and Kenji's mother. If Haruto has a son named Daichi and a daughter named Hina, how are Daichi and Hina related to Aiko, Kenji, and Yuto? Is Mariko their aunt?
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