Plot
Raph, Donnie and Leo are out for another night mission/patrol which leaves Mikey at the lair as with Lillie who unfortunately caught a cold, making Mikey the big brother and having to care for Lillie until his brothers return.
This takes place a few months after the invasion to give you a better general idea

Mikey was in his room and he sighed a bit as he swung his legs back and forth in his bed, looking rather bored as he remembered why he had to stay home tonight since it was his shift to watch over Lillie. He gave a soft huff while he glanced over to see his journal on the bed next to him, normally he'd draw or sketch whenever he was bored but he needed to have some focus on caring for Lillie in the lair.

"Maybe I should go check on her, Lillie's been kinda quiet.." Mikey sat up and he got out of his bed as he got out of the train car that was transformed into his room, each of the brothers had gotten a respective car to decorate and change into their own rooms however they pleased, after their original lair was destroyed by the Shredder, they had to find a new place to stay and hide. It took a lot of work for all of them but eventually they got their new place cleaned up, it felt just like home again.

Mikey made his way to where Lillie's room car was as it was across from Donnie's room car and he heard small sniffles and wheezes as worry laced his features and he opened the door to the train car, he saw Lillie in her bed shivering and trembling as her cheeks were a slight red hue as the orange masked brother went over with concern. He carefully nudged Lillie but noticed that she didn't wake up as Mikey panicked a bit. "Lil's..! Come on, wake up! Please!" Mikey said, Lillie whined quietly but started to open her eyes as she saw Mikey and she was trembling as the poor redhead had chills.

"M..mikey, it's so cold.. why is everything spinning?" Lillie mumbled, Mikey lightly pressed the back of his hand against Lillie's forehead and he noticed her temperature wasn't normal in fact it felt like she was burning up. "Lillie, you're sick. You're practically burning a fever.. okay, it'll be fine I can handle this. Big brother Mikey is gonna take care of ya!" Mikey stated.

Mikey got a more breathable blanket for Lillie and he put her back in bed as he put the blanket over her, he then dashed off to grab some other supplies he needed. He came back shortly after with a small bottle of cold and flu medicine tucked under his arm, a washcloth and a dish full of water.

Mikey set the dish and washcloth down on the nightstand as he grabbed the bottle of medicine and poured the correct amount needed into the cap as he held it out for Lillie but the child turned away from the medicine, clearly not wanting to take the rather nasty tasting medicine. "I know Lillie, but if you don't take the medicine then you can't get better. Come on, it'll be real quick." Mikey said.

Lillie nodded a little in understanding and she managed to sit up despite her feeling dizzy, she was given the little medicine cup and she quickly drank the liquid medicine as she made a whine when it was gone. "See? It's over, now just lay down and get some rest. Leo says that rest does a weary body good." Mikey said, gently grabbing the now empty medicine cup from Lillie as the redheaded child laid back in her bed.

Mikey set the cup aside and then began to lightly dip the washcloth he grabbed into the dish of water, wringing out any excess water from it so it wasn't soaked but damp enough to help Lillie cool down.

He laid the folded washcloth over Lillie's forehead and he saw her flinch but relax as Lillie gave a small sniffle but slowly started to doze off, Mikey left the room quietly so his little sister could rest. He knew all too well how Lillie was feeling, he had been in her position once as a turtle tot where he had caught a rather high fever to where it made his brothers worried beyond belief.

He wasn't going to let his worry and fear consume him as he had to keep calm and help her recover. Mikey then went to the kitchen and he got to work on making something for Lillie to eat when she woke up.

After about an hour passed, Lillie began to shift in her bed and she started to wake up as the washcloth was warming up since all the cooled water from it was dissipating. Lillie looked around and she then sat up but her dizziness was finally gone, she set the washcloth in the dish of water and she was still a bit warm from the fever luckily her fever had subsided enough but not entirely for her to feel completely better.

Soon she heard a light knock at the door and looked over to see it open, Mikey walked in carrying a bowl of soup in one hand and under his arm was a bottle of water he brought for her so the child could stay hydrated. "How are you feeling Lillie? Any better?" Mikey asked, he walked over to the bed and he sat down on the edge of it while Lillie rubbed at her head a little. "I don't feel dizzy more but, I still feel all warm.." Lillie replied.

"At least your fever came down even if it was for a little bit. Here, I made you some soup and I got you some water so you can keep yourself hydrated. Leo told me that when you're sick, you need to drink plenty of water to help your body feel better faster and Donnie says that it'll help your immune system work more efficiently." Mikey said, he had heard the twins discuss medical situations a handful of times plus he had seen Leo read from different medical textbooks he'd get from either up topside or from the Mystic Library.

Lillie thanked Mikey and she was handed the bowl of soup as Mikey set aside the bottle of water on the nightstand, Mikey gently patted Lillie's head and he left her be so she could eat. Lillie ate her soup quietly and she remembered how her parents would care for her when she was sick, she felt lucky that she had the brothers around to care for her.

Once she was done with her soup, she set the bowl on the nightstand after grabbing the water bottle and she took a few sips to try and rehydrate herself. She put the water bottle on the nightstand and decided to get some rest as she laid back down, dozing off a little as Lillie nuzzled her pillow a bit.

Timeskip

After a good four hours or so Raph, Donnie and Leo came back home and they stretched as Donnie looked exhausted after the mission and Leo tiredly rested his arms behind his head while they walked. "You think Mikey and Lillie are okay while we were gone?" Leo asked.

"I'm sure they're both fine, Mikey is technically an older brother now so he can handle taking care of Lillie no probbles." Donnie reminded, he looked over at his twin and Raph went to Mikey's room and moved the door but saw Mikey wasn't in his room which made him arch a brow bone. "Mikey ain't in his room." Raph said.

"Maybe he's in the kiddos room?" Leo suggested, the trio walked over to Lillie's room and they found the most wholesome sight. Mikey was in a chair and he had fallen asleep next to Lillie's bed as he gave a tired hum, Lillie was fast asleep in her bed and she hugged her plushy close to her.

Donnie took notice of the medicine bottle and dish of water on the nightstand but got a general idea of what happened. "Let them sleep, seems like the two of them had a bit of a harsh night." Donnie said, Leo agreed as well as Raph and Raph quietly went over as he carefully picked up Mikey and took the orange masked turtle back to his room so he could sleep more comfortably.

Raph tucked Mikey in bed and he left him be so the younger brother could sleep as Leo went back to his room so he could sleep as Donnie went to his room to recover as well and Raph did too but he made sure to listen for anything incase Lillie woke up or if anything else happen, he was pretty proud of Mikey taking care of Lillie when she was sick.