ミクちゃんがルカ姉によしよしされてニヘッて笑って、それを見て可愛いなぁってニヘッてなるルカちゃんがすこです。

Miku-chan getting headpats from Luka-nee and going "nihe" while smiling, Luka-chan seeing that and then also going "nihe" while thinking it looks cute. It's amazing.