According to Retskrivningsordbogen (official Danish dictionary) there is no singular form of sibling. I think you'd be better off saying "en søster eller en bror". The answer from sproget.dk in the link says that "en søsken(d)" is used on the internet (and that this is also the spelling of the word in Norwegian) but it is not a part of the dictionary (yet).
Again did not get to read the last word and got approved as correct. A bit irritating.