Based on the early character settings, Diana doesn't need to be jealous of Liz because she's the more outstanding one among the sisters. She's taller and more voluptuous than Liz, and unlike Liz, who is a housewife, Diana is a successful career woman (she's probably a company executive).
Zetto clearly designed Diana based on the character Mijung Kim from a Korean comic, and she also has a family and a beautiful daughter with glasses. I think she simply has some sex addiction; she's bisexual and has strange desires for her sister. However, early sketches did show scenes of Liz, Diana, and an unnamed man having a threesome
Sometime in 2024, Zetto considered leaving Diana's story, but she lost the vote to Liz. Diana has her own storyline; at least, having sex with her nephew was one of the early plot points. Perhaps she'll be the first to have a relationship with Ranta. Besides that, she'll also have sex with an older man in the company and a black man.
However, the tricky part is that according to the current setting, Yuki is only ten years old. Having a sexual relationship with such a child would always cause some trouble. I wonder if Zetto will change the setting.
Finally, I think it's very interesting for everyone to immerse themselves in the story and discuss it. Perhaps the inspiration from that discussion will even help Zetto's own writing. He does sometimes consider reasonable suggestions from fans, such as the storyline involving Yuki and Diana
