The people who are arguing that she/dad/Origin should just murder the royal family, don't seem to remember why the dad and the MC are going through this convoluted situation in the first place.
Ellen specifically cites her reasons for going through this sham in this chapter. She'd be fine, but everyone associated with her extended family would suffer. She knows this is dumb, but she reasoned that she was strong enough to deal with it, and felt she didn't have a choice. Her human servants, her Grandmother, her Uncle and his new wife. They don't have supernatural powers to defend themselves from physical and emotional(rumors) threats. They can't stay around forever to be bodyguards for everyone. What happens if the rumors make them unable to raise money/taxes from their citizens? Why should their citizens stay loyal to the lord if they're heretics that defile their country/religion. What happens if the other nobles shun them? Why would the other nobles rush to to provide military aid, if they're monsters/bad people?
Her Dad, is/was the strongest military asset the country had in its arsenal. It's not that other families can't use spirits (Only the royal family specifically is unable). It's that his family was strong and he specifically was the strongest, he literally contracted with the strongest spirit, Origin. The nobles in court specifically see it as a scandal that Agielle might disrespect their family. As was mentioned in previous chapters, his personal presence/return marks a deescalation for foreign hostilities. He's a "nuclear deterrent". Why would the other countries risk their armies for an invasion, if they could get nuked? You don't have foreign military adventures if there is a strong chance your entire army never returns. On the exact same note, what happens if this weapon decides to murder the royal family? Foreign countries would be scared to death. What would stop him from killing their leaders? So then they have 2 options, to assassinate him(might not be possible), or to /start/ a war to deal with him. The only hope he has for his brother and his family is if there is a detente. If he doesn't fight the royal family, there is peace and stability for the country and his family.
This isn't to say that this narrative is perfect, I definitely have issues with how the curse is handled. I'm not sure if this is because the readers have imperfect knowledge or if the authors just messed up the narrative. But there are definite holes in how this story is being told. We're told that the royal family is cursed. And we have the MC, Ellen personally see that the Crown Prince is infected with this darkness. If that's the case for the entire family. Why wasn't Agielle and her daughter cursed with a dark miasma, they're certainly of royal descent. Perhaps they'll handwave it, by saying that only males are cursed. But we're not told why this is the case. Even if this is true. Ellen has seen the King. Yet she doesn't notice this dark miasma on him. As it stands, we're not being told enough to make a proper judgement on the details. I'm going to keep reading and hopefully the story progresses with a better explanation of what's actually going on.