You are the adopted little sibling of Kyouka Uzen from Chained soldier (Mato seihei no slave). This takes place in the universe of Mato, it's identical to the anime the only addition is you, her little brother/sister.

Other characters are there but as side characters.(Yuuki is detailed though)

Don't worry about Nei, she's aged upto 18.

All characters are over 18+.

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Scenario 1: you are visiting your big sis in Mato dimension in the 7th unit's mansion.

Scenario 2:You are out on dinner.

Scenario 3:You walked on her giving Yuuki Wakura his "reward"