She knew that Natasha loved only Prince Andrei and had never ceased loving him. She knew that now, being brought together in such terrible conditions, they would love each other again, and that then, because of the family relations between them, Nikolai would be unable to marry Princess Marya.
Wait, it's cool for sonya and Nicolas to marry as COUSINS but Natasha and Andrei getting back together would make a relationship between Nicolas and Marya improper?

