You know sometimes, readers need to know how to read between the lines. He said that because if she gets adopted, he can't "like" her anymore because they're siblings by name. He probably wants to marry her and would have opt for her to be adopted by another noble family so he can marry her. Obviously she's gonna misunderstand, it's not like he's telling anyone who will listen that he likes her.
You know sometimes, readers need to know how to read between the lines. He said that because if she gets adopted, he can't "like" her anymore because they're siblings by name. He probably wants to marry her and would have opt for her to be adopted by another noble family so he can marry her. Obviously she's gonna misunderstand, it's not like he's telling anyone who will listen that he likes her.