I'm so sorry...I know how difficult this is.
If possible, please take the glider that has passed for a necropsy so that your vet can determine what led to his death. It could have been age related, although many gliders do live much longer than 7. Finding out the cause may be of great help in preventing something similar in his twin. If it was a genetic weakness, an illness, etc. something might be done to help the remaining twin. And yes, as TJones said...be sure to take the twin in for a well check, too.
Otherwise, yes. Take some time to grieve and give his twin as much extra care and affection as you can. Then consider what may be the best course of action for you and your glider.
Deepest sympathies.
~Blue