A/N* The nickname Maggie has no reference to Maggie Smith, and is merely a coincidence, purely a joke name used by Sirius.
McGonagall's around the same age as Sirius and Remus in this because it's fanfiction and well I want her to be! :)
Set during the events of OTP.

Sirius and Remus sat at the kitchen table of the dank and dusty Grimauld Place sipping cold butterbeer. It was a sweltering glorious day outside, however due to the Minister for Magic's insistent calling to the whole wizarding world that Sirius was a notorious mass murderer, he had seldom seen outdoors since his return to the family home. Remus didn't have the heart to leave him on his own anyway; and so chose to stay inside with him for company.
Several minutes comfortable silence passed before Sirius's boredom became apparent. He absent-mindedly gnawed on the lip of his bottle-a trait Remus noted that he carried from his animagus form.
Just then, emerald flames erupted in the kitchen fireplace as the tall, slender frame of McGonagall dusted clean her travelling cloak and stepped out from the grate.
Remus was sure that if Sirius had a tail at the present moment it would be wagging profusely. Any company was welcome company from him at the moment.
"Minnie!" Sirius cried happily, stepping over the bench with a wide grin.
"Now Sirius, you know I prefer Minerva." She mock scolded with a matching grin.
Remus was sure that if he had a knife, he could cut through the sexual tension between the pair. The only problem being Minerva was too proud, and Sirius was too bloody dim to realise there was any.
Several minutes of heated discussion surrounding Hogwarts, Harry and the Order later the trio fell into silence. Minerva's impatience was apparent first as she asked rather huffily, 'Is there nothing to do around here?'
The two men shrugged at her and went back to their butterbeer as she huffed and grumbled.
"When I was little, I used to try and find pictures in the wood grain." Sirius offered pointing at the table.
Upon Minerva's confused expression he elaborated, "You know, like in clouds and stuff."
The three immediately took to staring at the table trying to find pictures for something to do.
After a minute or so Sirius let out a snort gathering the other two's keen interest.
"This one looks like you naked!" Sirius chuckled gesturing at Minerva.
Remus flushed crimson as Minerva dived across the table before sitting back with a smug expression.
"Nonsense," she started looking at Sirius. "I'll assure you my body is much moreā€¦toned." She finished her smile widening.
"Pfft OK then" Sirius said looking down with a scrutinising expression letting out another soft snort.
"Saggy Maggie" he mused to himself quietly with a smile.
However Minerva's animagus hearing caught exactly what he said.
Sirius let out an almighty yelp and several choice swearwords and the Shearing Jinx Minerva fired at his prized hair.
Remus laid his head on his arms and sighed at the play fight that ensued between the couple.