Michael is not the oldest angel. He has an older sister, the most powerful archangel of all. She isn't known very well considering she fell when Castiel was born. She did have a good amount of years with Michael, Lucifer and Gabriel though, and the three secretly missed their big sister.
One day, she surprised them all.
Michael and Lucifer were tossed into the bunker after being released from the cage a couple weeks ago.
"Hola boys."
The two spun around. Michael looked at his sister before tackling her with a hug. "Andrew!" He cried. She laughed a little as her other brother joined the hug.
"Miss me?"
"Is that a question?!" Lucifer shouted. Sam, Dean and Castiel took a step away in shock.
"Hello Cas. You probably don't remember me. I'm your oldest sister, Andrew. I fell when you were a newborn," she explained. Castiel nodded slowly. "Long time no see, Motor Mouth," she chuckled to Michael. "And you too, Tumbleweed."
"I don't get it....." Castiel muttered as both of his older brothers went pink in the face, knowing the answer.
"This one never shut up," Andrew motioned to Michael. "And this one fell everytime I tried to teach him how to fly. It took months. Two. Months."
"Quiet," Lucifer blushed. Dean and Sam were grinning at the brothers' embarrassment.
Gabriel appeared and tackled Andrew with a hug. She fell to the ground in shock before smiling and kissing his hair. "What's up, Sir Pranks-A-Lot?"
"How the hell do you remember our nicknames?!" Lucifer asked.
"How could I forget? That was all I was called for at least 500 years," Andrew chuckled. "Motor Mouth, Tumbleweed, Sir Pranks-A-Lot and Squid."
Dean snorted. "Squid?"
"Don't ask, we'll be here forever," Andrew chuckled. "So, boys, how've you been?"
"Really?! You fall, we don't see you for trillions of years, and that's what you come up with?!" Michael cried. "How about 'where the hell have you been when we were at war?!!'!!"
"I was doing business! Don't get into this Michael! You can't!" Andrew shouted.
"I can do whatever I want!"
"Not this!!"
"Why?!!"
"Because you're my baby brother and I'm not letting you suffer the way I have been for the past trillion years!!!" She screamed, shaking the walls. Sam and Dean grabbed onto the furniture to prevent from falling. "I know what I did was wrong but, hell Mike, give me some leeway! I've been stuck with the Darkness!! The only reason she got out is because she almost killed me!! I'm not putting you in the line of fire Mike!"
"I'm the best warrior God has!"
"I DON'T CARE! I LOVE YOU TOO MUCH TO HAVE YOU DOING WHAT I DO!!!!!" Andrew's eyes glowed white and Michael took a step away, knowing he passed a line. She tried to control herself before looking at Michael with strained eyes. "I'm not losing you the way I lost my family."
Lucifer stared at his older siblings while Gabriel took a step back.
"Andrew..." Michael said in a soft voice.
"They got in the way and I couldn't keep the darkness away.... I had a son and a daughter..... my son was eight, my daughter twelve," Andrew confessed. "I refuse to lose more of my family."
"Thanks for the sad fest," Dean clapped his hands. "I'm going to get food. Sam wanna-"
"Let's go."
They hurried out the door and Andrew stared down -er up, because they were all taller than her, but the look she had made them feel two feet tall- at her brothers.
"Alright. I'm done telling you about my life. Brighten up, Motor Mouth," she poked his side and he flinched. "Aww, is little Motor Mouth still ticklish?"
"I am not!"
"Reeeaaaallllyyyy?" She smirked, tackling him to the bed and holding him down with her grace.
"Andrew this isn't fair!!" Michael whined. "You're stronger!"
"You're stronger than us! You used to pin us and tickle us senseless!" Gabriel argued. "Castiel come here! Watch Andrew and learn."
Andrew flicked her wrist and Michael's arms were above his head. She started to gently tickle under his arms and Michael twitched, pushing his lips together. "Come on, Motor Mouth! Laugh it up! I haven't heard you laugh since we were kids!"
"Never!" Michael gasped.
"Get him good Squid!" Lucifer smiled. Andrew got an evil gleam in her eye and at that moment Michael knew where Lucifer had gotten all his evil from.
"You know, Tumbleweed, you can always help. You too, Sir Pranks-A-Lot," Andrew smiled. "And I guess you too, Puppy."
"Who's Puppy?" Gabriel asked as Michael felt his lips trembling.
"Castiel of course! He looks like a puppy sitting over there," Andrew explained. She whipped out her dark blue wings and stroked Michael's underarms with them. The younger angel shrieked before dissolving into loud laughter. "Hey Tumbleweed, get his feet!"
"Nonononono! Lucifer dohon't!" Michael begged.
"Hell yeah!" Lucifer started to scribble his fingers across Michael's foot and Michael's laughter went silent.
"Poor little Motor Mouth is about to die," Andrew teased. Gabriel laughed gently.
"I know where Puppy gets all his ticklishness from!" He cried. Castiel stiffened.
"I am not that ticklish," he argued. Gabriel tackled him off the bed and started to tickle his stomach, getting very loud laughter to match Michael's.
"SQUIHIHIHID PLEHEHEHEHAAHAHAHASE!" Michael laughed, trying to pull his arms down to protect his armpits. Andrew took away the wing and blew a raspberry under his arm. Michael screamed so loud the bunker shook. Lucifer grinned at his sister.
"Michael did that?!" Lucifer cried.
"I'm a genius!" Andrew chuckled. Finally, she did slow down her tickling and kissed Michael's forehead. Gabriel had shown mercy when Castiel jerked his arms down and punched him, so they were sitting on the couch. Andrew got up to join them and Lucifer followed. Michael found the strength to walk over to them before placing his head on Andrew's lap and falling asleep. Andrew chuckled and gently ran her fingers through Michael's hair. "You finally shut up," she chuckled, causing him to smile softly in his sleep. Castiel had also fallen asleep due to his tickling and the only one's awake were Andrew, Lucifer and Gabriel.
"I haven't seen you in forever. We learn your family died to the Darkness, and you tickle the hell out of Mikey. That's gotta be a new one for the team," Lucifer chuckled. "What was our team name again?"
"Umm..." it took Andrew a second before she remembered. "God's Headaches."
"Oh yeah," Lucifer laughed. "We were his headaches."
Castiel sat up.
"Why do they call you Squid?" Castiel asked.
"When we were younger we were playing by a lake when a squid grabbed me. It pulled me into the lake and I was stuck feeding its young for hours at a time. Then, the squid started playing with me while I sat in the water and it lifted me high. I named the squid Jewel," Andrew explained. Castiel nodded.
"That wasn't that long of a story."
"I'm too lazy to repeat it."

I've been wanting to publish something like this for a while so yeah.