He's going to steal his sister's man.

Nikolai hates his sister. And she hates him too. It all started when she broke his family's old heirloom painting and blamed it on him, causing his relatives to hate him.

Now that they already hated him, why not go full on out? When his sister's marriage was arranged with the Grand Duke, the most handsome man in the country and probably in the world, he decided that he was going to steal the Duke and get his own happy ending with him.