Unfortunately, my subscription has just expired, so I don't have access to the story anymore and can't say exactly which episode it was. But it was in the Tale of a self-centered girl chapter from Treyse's perspective.
"Bill Arnold, the eldest of the Rockmann sons, came from his own domain to visit together with his wife. A few hours later, the parents of Sir Alois’s
stepsister, Melly Arnold, came for a visit, bringing flowers."
Original text: 容態が安定したアルウェスを公爵邸で引き受けてから、それはもう目まぐるしく、現在は自分の領地で生活を送っている長男のビル・アーノルドが妻を連れて見舞いに訪れたり、その妻でアルウェスの義理姉となるメリー・アーノルドの両親が数時間後には花を持って顔を見に来たり、 [...]
Alois doesn't have any stepsister, so 義理姉 should likely be translated here as "sister-in-law" instead of "stepsister". It has been previously mentioned in the story that Alois' older brother Bill is married. In the above passage, it sounds like Bill & his wife Melly first visited, then Melly's parents showed up later on.
Sorry if this comment is late. Thank you so much to the JNC team for all your hard work on this volume. I'm very happy that volume 4 was translated and thought the translation was truly excellent.