There is much playing with the timeline here, and it is very AU, but please bear with. It is set roughly at the beginning of Harry's Sixth year - ie they are at war already. Enjoy!
It was just a normal Order meeting until that point, and Minerva had no reason to expect that it would be any different to all the others. That is to say, of course, there would be the usual squabbles and arguments, Albus would attempt to keep control and the meeting would continue into the small hours of the morning. If they were lucky, one decision may be made. She sighed. She was really far too old to be doing this every other night, on top of her other duties as Deputy Headmistress, Head of House and second-in-command in the Order. The paperwork alone was horrendous.
Albus was present, which unnerved Minerva given his lack of attendance over the past few months. Fudge was keeping him busy at the Ministry more often than not. She was quite surprised to see him, and could only assume that something major had happened.
Or he had discovered what she had done.
She preferred not to dwell on that possibility, knowing the likely initial reaction to it. She also knew that it would mean complete exposure to the rest of the Order, which she would protest against far more strongly. It was unlikely Albus would do anything truly terrible even in his initial rage, and she could certainly hold her own in a duel with him.
No. Albus was not the risk of her actions were discovered. Mad Eye Moody was not either - he knew of her heritage, having been at Hogwarts when she had been. His paranoia would not extend to duelling her, even once he heard the news. The person she was most at risk of retribution from was likely to be Severus or Molly. She winced thinking about it. There would certainly be no friendly conversations for a while yet. She shook her head, trying to rid the thoughts from her mind. After all, there was no point worrying about something she did not know would occur. Albus didn't check up on him very often, and was not due to visit again until March. It was early November, and so she had many months to go until he would know for certain.
"I call this meeting of the Order of the Phoenix to order. We have some urgent business to discuss."
Minerva snapped back to the present, knowing that this would likely need her attention. She noticed a figure she recognised in the background, and was curious to see why he was here and why no one else had noticed him. He was no wallflower, and his stature was not the most conducive for hiding in the shadows.
"I have a new member to introduce, whom I am sure you will welcome. Minerva, I am sure you recognise him?"
The lightly teasing tone was present, which either meant that Albus was a good actor, or that one particular secret was to stay secret for another day. She nodded in reply to Albus, and watched one of her elder brothers step into the light.
"This is Otto McGonagall. He is an expert Arithmancer, and is also talented in the Dark Arts and their reversal. Please welcome him."
So he was using their mother's name as well. Interesting.
"It is a pleasure to join you, especially now I can embarrass my baby sister!"
His accent was still noticeably German though, and Severus shot her a look at it. The ball was firmly in her court.
"You seem to have forgotten Otto who the real power has always lain with. I may be younger, but you have yet to prove our parents wrong."
Her reply was equally light hearted on the surface, but the cogs were turning rapidly underneath. It would not take long before questions began to be asked, and she was not sure she could answer them all.
"I was unaware you had a brother Minerva."
Severus' voice was silky smooth, and held far too many undercurrents of danger for Minerva to feel comfortable.
"I have 9 actually, all of them much older than I am. Otto is the second closest in age, and he is 10 years older than I am."
Her reply was sharp, but her answer would only create more questions. She would make no promises about answering them. Somewhere across the room she heard a low whistle of disbelief, and mutterings about how people no longer felt so sorry for Ginny Weasley who only had 6 brothers.
"And German accent and first name, Scottish surname. Care to explain either of you?"
Severus pressed on, being the only one brave - or foolish - enough to ask. Otto glanced at her before nodding, an indication that she was to continue.
"My father was German. I, and Otto evidently, prefer to use my mother's surname. As I said, I am much younger and I spent much of my childhood in Scotland. I was the only one educated at Hogwarts. My brothers received a mix of private tuition and schooling at Durmstrang. Consequently, they all have German accents. If I bothered to try, I speak German or I am in Germany I also have one, but that happens less often. My middle name is German though - Annaliese."
Minerva hoped she had answered enough questions, although she doubted it especially due to the cagey way in which she had referred to her father. And then she caught sight of Albus' expression and knew the game was up. She might as well admit it when he asked the inevitable question.
"I have one other question for the Order." Albus interrupted the line of questioning. "I happened to visit Nurmengard today, and found that Grindelwald was dead and buried, perhaps a month ago. Minerva?"
She nodded, confirming his suspicions.
"Why did you kill Grindelwald?"
The outraged gasp was almost more than she could stomach. Minerva McGonagall a murderess?!
"I wished to kill my father painlessly before Voldemort could find out more information from him, and given that you would never do it yourself, I did."
With that she stood and left the room, Otto following her.