There is not much, that MC can do. The emperor is effectively blackmailing him. I do believe he does it for his sister and I do believe he is going to let them go in the end, but he is still an extortionist and you don't truly believe anything they say and you tell them anything they need to hear. The wisest course of action is, MC should try to earnestly create a cure, but also prepare for a sudden departure just in case. And should that Hero set him free after he cured his sister, he should go. Take that world tree seed. No vengeance. Just go. And never return. Ever! That nation is not an ally. And should they ever cross paths again, MC should better be prepared. And that Emperor better, too.