Can someone give a spoiler?

Because she's poisoned and bedridden and he needs to expand his influence somehow by using her and stuff and that 100 day whatever stuff prohibits them to act as they wish so they need her to climb to society while they are quiet. I guess? I don't know but that's what I understood so far (〜￣△￣)〜

But they did mention that it was a long time plan right? I think there's more behind it.

I was also wondering why does the mc looks like the grand dukes daughter and how did they know she exist?

I also thought about that too, cuz there's no way that they just coincidentally look like each other and stuff