All because she couldn't reach out and take his hand that night and run away with him w/ her sister... She had to drug him, sleeping w/ him and put the lives of many in jeopardy for a moment of pleasure and greed... Now he has to risk everything to save the best girl, a random girl w/ integrity, and this woman whom the author also wants to be included w/ the harem.
Let's call her side concubine
ML would have happily rescued her sister from the palace, or her father or whoever had seized her at any point in the story. He had already grown attached and protective of her. But XueYun threw it all away, threw it all away repeatedly during the story because of her pride. She's infuriated me repeatedly because of her damned pride, always being jealous of Yuning in the background as well putting on the self-sacrificing role and looking pained as ML and Yuning go off in a different direction.
Which annoyed the piss out of me she betrayed Yuning by drugging him w/o consent. Xueyun and Yuning were close friends, both loved ML and could quite possibly share him with each other at the outset. Yuning was also accepting of the fact that ML wasn't the biggest jewel in the ML's eye. Jealous yes, but accepting none the less.
But Xueyun's betrayal, I see a betrayal arc between the two while ML is out risking his ass trying to save all three women from the palace now.
But Foursome now? Hmm...