There is no word “fætter” in Norwegian, but there is a word fetter 
And it specifically means “male cousin”. “Female cousin” is kusine.
Male and female cousins together can be called søskenbarn, literally “sibling-children”, which sounds like it should mean nieces and nephews together but for some reason doesn’t. Some women refer to their nieces and nephews together as their tantebarn, “aunt-children”. I’ve never heard a man talk about his onkelbarn, however.