While the two of them spent some quality time together, other people were spending quality time to destroy them as well. Diana already prepared her plan to deal with Cayenne. She's got so many plans in mind depending on which route will Cayenne choose.
In their family, they were trained not just to become a fine lady, elegant with aristocracy but, they were also trained to be ruthless and cold. She grew up in a cold environment where even families can be slaughtered if they won't follow the rules. She was walking on egg shells and the day has finally come.
It would be easy to kill her grandfather but she didn't want that to happen. Everyone in their organization and business trades still recognized him as the head of the empire, if he suddenly dies, they will be the first suspect. She can't deal with too much attention on them. So, all she could do was the cleanest and fastest way – make sure that Cayenne won't fight for the inheritance.