I have see a lot of people not understanding the motive behind their action. That because the du sister lied a lot and talked about grand ambition, when she's just petty.

Du familly true motive is simple: it's money.

The moment they get money from their betrayal, they immediately used it. Not waiting the deal is done, immediately.

They are gold diggers. They want wealth. They want to bleed a powerful family dry. And their plan could have worked. Cuique as Jinqi’s concubine, and Zheng as Xiuwan’s husband.

Why they betray ? because the plan worked only for Zheng. So, they decide to target a different family. And money. They got their share of it.