Fred and George could have been at the Department of Mysteries
With the Carlin Brothers having gotten to the Department of Mysteries in their re-read, it's got me thinking. It would have been so cool if Fred and George had gone to the Department of Mysteries with the Order members.
Let's say they had just closed the shop and gone to hang out with Sirius for a little while. Since they're not living at the Burrow, Molly would have no say who they get to go see. And they love goofing around with Sirius. It's also around the time that Remus, Kingsley, Tonks, and Moody get there, right?
So Snape gets in contact with the Order, confirming that Sirius is at HQ, and letting them know what happened. Fred and George are determined to go, because their little brother and sister and the other kids are in danger. The others can say that it's dangerous, that it's Order members only. But they're now 18, AND have left school. They're officially eligible, and their first meeting would have been coming up as soon as Dumbledore called another one.
I don't see them really doing much, other than trying to round up the kids. They would probably miss Sirius's fall through the veil, since the other kids aren't there either. They would help in rounding up those incapacitated Death Eaters and bring them to the veil room for the ministry workers. Maybe do a bit of healing on Tonks (they do seem to have a knack for healing themselves from dangerous tricks, no?)
I just think that it would be cool to have the bridge from DA member to OOTP members there.