There is a wonderful support organization in the United States called Twinless Twins who offers support and resources for multiples of all ages who have lost their co-multiple at any point in their lives. They can be reached at http://www.twinlesstwins.org They have support Chapters all over the United States and an annual convention, sometimes in Southern Canada as well.
There are also several books about being a twinless twins (a term used even for loss within a higher order multiple set of multiples): Living Without Your Twin by Betty Jean Case; The Survivor, by Lynne Schulz; Who Moved the Sun? a tribute to the loss of a twin brother; and Twin Loss by Raymond William Brandt. As well, my Site at http://www.jumelle.ca has articles and resources for survivors who might appreciate them.
I am so sorry for your loss.