What is the name of this manga? There is a younger step brother who everyone thinks is an angel. The step sister finds him smoking, but nobody believes her and the two eventually get romantically involved

Okay this won't help you but there are so many manga with this kind of premise that it's kinda cliche and you'll have to be more specific to what you are referring to. However, this sounds like a smut manga so you may want to try looking under that tag.