My sister is already insecure with her looks, so I'm thinking of installing a funhouse mirror in her room.
That's not very kind. What is your reasoning for wanting to make her feel bad?
Now I'm sorry for saying that because it seems like you don't like to joke around. Maybe we should talk about something else.
It is cruel to make someone feel bad about their body. You should try to support her and get her help if she cannot overcome her feelings.
Yeah, good point. I need to have a talk with her.
While you might have been joking, making lights of someone's feelings of inadequacy is mean and you should be supportive rather than bring them down. I'm glad you realize that you that talking to her and lifting her up is the better course of action.