https://www.dmm.co.jp/dc/doujin/-/detail/=/cid=d_379651/ --------------------------------------------------------- Please do not use MTL 機械翻訳は使用しない方がいいです
If anyone is curious, the author made it non-NTR by making The MC and the step-sister accepting the new girl out of kindness. This is but a teaser, pretty much like the author's previous work, to the upcoming doujinshi which will be released this month (May) so not all details are revealed. I personally think 30th day orgy isn't the new girl's first time with MC. It's just being shown in full that day in order to keep everyone's anticipation for it. Since there are other girl involved with the MC, it's pretty weird direction for the author to take after what had been established in the original work, and after he stated there's no NTR in the stories though.