Oes gennyt frawd neu chwaer? Do you have a brother or sister? This is in my dialect, I'm from mid west Wales, Ceredigion County. We have the most "neutral" dialect people say! Gobeithio bod hwn wedi fod o fudd!

You could also say: Oes frawd neu chwaer gyda chi? Chi is more formal and gyda might be used instead of gan.