The Marauders and A Third Black?! (James Potter x reader)

The Noble And Most Ancient House Of Black

"Master Regulus sir!"

Regulus woke with a jolt, and looked around the room

"Yes Kreacher"

"Master Orion wants you downstairs in five minutes"

"Oh, okay! Tell him I'm on my way Kreacher"

"Yes Master Regulus"

And he was gone with a loud CRACK.
Regulus got out of bed, put on some clothes and went downstairs where he was met by both his parents who looked to be in a hurry.

"Regulus, your Sister will arrive today at eleven and she has an appointment with the headmaster of Hogwarts at twelve to get sorted, I need you to remind her"

"Why can't you do that yourselves?"

"Because your Dad and I are going to your aunt Druella and help out with a few things for some days"

"Okay then!"

"Good!"

And with that they disapparated.

Regulus had completely forgotten that his older sister, and Sirius twin was supposed to come home. Since she had been expelled from Beauxbatons, Sirius hadn't known at all, apparently he was forbidden to write to her ever since he was sorted into Gryffindor.

But Regulus knew he'd been writing a little bit with her at school. Regulus was excited to see his sister, he had been allowed to write to her which he had done.

He had to admit to himself that he was nervous about her meeting with the headmaster professor Dumbledore, that's when she would be sorted. And he had a feeling she wouldn't be put in Slytherin.
He decided to read until she'd arrive.

After a few hours Kreacher said he had to go to get Miss Black, and then he heard a loud CRACK in the hallway. He slammed his book shut and ran out to the hallway

"LYYRAAA!"

"REGGIEEE!"

"I've missed you soo much"

"Me to kiddo! Woah your very tall"

He laughed at his sister.

"Sirius isn't here I'm guessing?!"

"Nope, he's at the Potters, mother and father never told him you were supposed to come home and start Hogwarts"

"Oh, that explains it"

"Miss Lyra! Kreacher is to remind you that you have a meeting with the headmaster of Hogwarts in an hour"

"Oh, right! I'm getting sorted before term. How am I getting there Kreacher?"

"You'll take the flooo network, it has been opened for a small amount of time so you can go to his office and get sorted and then come back here. Kreacher thinks you should go up to your room and begin to unpack"

"Right, our room, how much has he changed it?"

"Oh you'll see" Regulus said with a smirk

They walked up to Lyras and Sirius room together, Regulus helped her unpack untill it was time for her to go and get sorted.

Regulus had gone down to the kitchen to wait Infront of the fireplace for Lyras return. It took a while so he stated pacing back and forth.

Then he heard something from the fire and out stepped Lyra.

"So?!"

"Gryffindor" she said looking worried

"Oh" was all he could say

"They're going to kill me right?"

"No I'll go and Talk to them they're at Druellas"

"You sure?"

"Yeah, 'course"

He turned to Kreacher and asked him to apparate with him to his mother and father.

Lyra sat down and waited, and waited and waited.

All of a sudden Kreacher was back, but without Regulus.

"Kreacher is to inform Miss that, her parents will not come back while she's there, they're very disappointed and that Miss is a disgrace to the Black family. And that she will stay here until the first of September when Kreacher will apparate with her to platform 9 3/4 where she will get on the train by herself and she is forbidden to speak to Master Regulus"

"What?! I'm going to be here by myself for a month!"

"Yes, Kreacher was also told by master Regulus to give you this letter"

She felt her eyes water as she took the letter and ran up to her room to read it.

She opened it and saw her brothers neat handwriting

Dearest Lyra, I am so sorry that I cannot come back. Our parents got furious when they got the news that you were sorted into Gryffindor, they think "you'll end up like Sirius". I tried to reason with them but it didn't work, I'm so so sorry.
I am forbidden to write or contact you in anyway and apparently Lucius Malfoy will make sure of it when we're at Hogwarts. (He's dating Narcissa now, poor guy)

When you get on the train, look for a compartment with a red headed girl about your age, her name is Evans and she's in your year and house, she is a muggle born, but she is nice and she will take care of you.
Try not to tell people your last name otherwise they'll judge you before they've gotten to know you.

When you get to your common room, try to find Sirius and explain everything. He'll understand.
I will try and write whenever I can but I cannot make any promises.
Please be kind to Kreacher.

Lots of love,
Your brother Regulus.

She'd read the letter about five times while crying and after a while she fell asleep.

Kreacher came to check in on her, saw her face was swollen from crying and remembered his promise he'd made to Master Regulus. "Please take care of her Kreacher, you're all she have now", so Kreacher put a bottle of water on ger bedside table and put a blanket over her.
Then he started planning what food to make her and made a reminder to ask Master Regulus about her favorite food.

He also made sure she would have things to do during the days so she wouldn't feel so sad.
In other words he would try to make her stay as good as it could be, like he had promised Master Regulus.

He would not let Master Regulus down, he would not break his promise to master Regulus.