There's a special bond between twins that other siblings just don't have. Twins are, even to the naked eye, generally closer than other siblings. It makes it hurt just all the more when essentially what is the other half is lost.
I'm sorry for your loss, and may the funeral go well and honor him justly.
“Nobody trips over mountains. It is the small pebble that causes you to stumble. Pass all the pebbles in your path and you will find you have crossed the mountain.”
- Author Unknown