Now that she had Chang, life wasn't too hard even without Gu Chengyi.
After all, if Gu Chengyi were alive, he'd control the soap dividends, but if he died, she might get a share. That's better than Gu Chengyi leaving all the Silver for the second son to study.
Even if the second son doesn't agree, it's not up to him. Chang and the third son are also Gu Chengyi's sons.
With Gu Chengyi's death, the second house naturally splits, and it's not the second son's decision to make.
Mrs. Li's motive is, of course, because of Mrs. Wang. If Mrs. Wang and Gu Chengyi were entangled, she might have harbored resentment and possibly even sought revenge against the two.
Otherwise, why would Mrs. Li enter that room when Mrs. Wang and Gu Chengyi were together?
This way, both of them are suspicious, but Wang Yue's motive stands stronger.
There's no way Gu Chengyi would willingly take so much medicine; wouldn't the medical hall be afraid of taking responsibility?
