https://www.dlsite.com/maniax/work/=/product_id/RJ01535163.html
https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_715289/
https://www.melonbooks.co.jp/detail/detail.php?product_id=3387915

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Given he was seemingly elementary school age when he met his sister in law some time before marriage, etc, his niece being only slightly smaller than him feels extremely odd, as if they were only a year or so apart, yet he seems like hes in highschool now (rather than being an adult), so that means she might not even be in elementary school (although probably is). Either shes tall, or hes short.

Base +17