It's not automatic that all dogs from the same litter wil have the disease. See the chart under 'Trait of Inheritance' here to see an assessment of the probabilities:
http://www.laboklin.co.uk/laboklin/showGeneticTest.jsp?testID=8158D
You'll see from that link that the lab would charge you or your vet £66 for the test. On top of that you'd have to pay your vet's fees for taking a blood sample or you'd have to obtain swab samples from the inside of your dog's cheek yourself. (Most dogs probably won't take kindly to having the inside of their cheek rubbed hard for 20 seconds, so getting a couple of samples might prove to be no easy task!)
You need to think about how you might respond to the results. If the DNS test shows that your dog's in the clear, you'll be able to breathe a big sigh of relief but what if you find that your dog might develop the disease? It won't improve your dog's life but will it make your own better or worse? (i.e. do really WANT to know?)