Zack can't say Rosie's name, so he calls her "sister," but he can't really say that either, so it ends up sounding like "teetee."
This afternoon, I told Rosie it was quiet time while I put Zack down for her nap. She got upset about that and closed herself in her room, pouting.
When Zack saw this, he said, "Teetee sad. Knock knock," and went to go knock on her door to see if she was okay.