Things that can't hurt you are harmless. It might be hard for you to believe her when your sister tells you that her enormous pet snake is completely harmless.
If your friend is scared of dogs, you can reassure her by saying, "Sure, he's 150 pounds and has a loud bark, but Fido is harmless!" Things that are safe or benign don't cause harm, and are therefore harmless, like your dad's harmless teasing or a compostable paper plate that's harmless to the environment. The word harmless originally meant "uninjured," or "not harmed," and came to mean "undamaged" by the end of the 1300s.