Indeed, Lady Tang and the Doctor had secretly conspired to poison the new dress of Xuexue so that she would fall ill and miss the visit to the Imperial Palace.
By then, they could take the opportunity to suggest to Situ Yongjie that Qinger should attend the banquet in Xuexue's place since they were sisters, and it would make no difference to the General's Mansion which of them went.
For the sake of face, Situ Yongjie would surely agree.
But man's plans are no match for heaven's.
Unexpectedly, Xuexue did not get poisoned, but instead, Madam Xie did.
Lady Tang, ruthless as she was, thought of Xuexue's motherly bond with Madam Xie and decided to kill Madam Xie first. She had the Doctor add slow-acting poison to her medicine.
Here they were, under the cover of night, conspiring how to poison Xuexue to prevent her from attending the banquet at the Imperial Palace and to have Qinger go in her place.