For example: “Of course, I'll help you! What kind of a question is that? I'm your brother, for goodness' sake!”
I gleaned some translations from the web but I'd like confirmation from you natives that you would actually use them in a similar context, i.e. you're surprised or upset that someone would even think of asking such a seemingly pointless question.
Was für eine Frage ist denn das?
Was für eine Frage soll das denn sein?
Was für eine Frage?
Was ist denn das für 'ne dämliche Frage?