Nana57:
When they were trapped Wang YAnJi said to CSS: you should thank the queen mother Xuan. If she hadn’t passed away, it would have been your eldest mother who died. That means she kept tabs and if the eldest mother died her family would have mobilise to return the coffin to the hometown so Wang YAnJi could execute her plan using CSS as a bait to trap LBY. The google translation of the novel sometimes is quite difficult to follow as it is literal and so the proper meaning is lost.
WYJ kept tab on CSS, she was intending to kill CSS's grandmother originally (by poison, I assumed, since there was a point that her grandmas was really sick and they had to call all her 3 sons back to be by her side). When her grandmother died, the family would return to their hometown for burial and mourning. WYJ planned to attack CSS on the way.
Later on, she changed her mind as the old empress passed away and CSS followed her wish to scatter her ash at the old house of the empress's father. CSS's grandmother recovered miraculously (since she stopped being poisoned).
CSS planned to visit Luo Yao on her way back after scattering the empress's ash. WYJ knew about this through Lou Li. She planned to kidnap CSS then as Luo Yao's post was near to her lair. However, LJT attacked CSS and CSG first, when they were on their way to the house of the old empress's father. Had LJT succeeded, it would have completely ruined WYJ's plan. Therefore, WYJ ordered LJT to be executed brutally.