The fact that Lan Wangji believes that his older brother will stop interfering in his life just shows how upset and off-balance he is. Lan Xichen has been interfering in Lan Wanji's life for pretty much all of their lives.

It's his brotherly duty.

And joy.

"I have heard there is this marvelous new invention, called the 'letter'." Lan Xichen notes, his voice droll. "You may have heard of it."