Yes, a third sibling was born moments later and then they were triplets. Although other solutions can explain this conundrum (for instance Bex's solution), this one (triplets) is a more common occurrence so is the generally accepted solution.

Well done to your brother!

Now there is even another situation where you can have only two siblings born minutes apart and not be twins when actually there were twins in the womb. This has occurred in real life but it is really rare. In this case, they were not conceived exactly at the same time: I will not keep you in suspense... here is the article:

http://www.foxnews.com/story/0,2933,322485,00.html