After Starlight solved their sibling rivalry, the royal sisters made a genuine effort to patch up the rest. Luna joined Celestia at breakfast, and she switched back to cooking the berry pancakes she first made, as it turned out pineapple on pancakes wasn't a good combination to Luna's taste buds. Being exhausted made it hard for Luna to have an actual conversation, but she did try and Celestia could understand her well enough to reply. On the occasion Luna really was too tired to join and would fall asleep with her head on her breakfast, Celestia would tell her not to worry about breakfast and go to bed. Then, laugh when Luna was out of earshot.
During the evening when Luna lined the halls with lavender, Celestia would join her if she happened to have some free minutes between duties. If she was preoccupied with dignitary ponies or another matter at the time, she'd try to catch Luna before nightfall to give her a small bundle of lavender, which always made Luna smile. She would suggest different aromas from time to time, like peppermint, and Luna was glad to use her ideas.
One night, Celestia had a meeting that finished much later than she liked. The time was just shy of 9 PM, meaning Luna was undoubtedly off to the dream realm by now. Exhausted as usual, Celestia heaved a heavy sigh, but she intended to still pick some lavender for Luna before she went to bed. She enjoyed they were really trying to spend time with each other and she didn't want to ruin it by saying she was too tired. But as she dragged herself through the doors, she almost immediately perked up when she saw one last pony waiting on her.
"Luna!"
Luna was sitting right in front of the doorway. She quickly explained she'd been waiting for Celestia's meeting to end and intentionally put off going to the dream realm tonight. The two ponies walked alongside each other to Celestia's bedroom, though Celestia didn't get in bed when they entered. Luna obviously needed something and Celestia didn't want to fall asleep on her.
"So, what is this about, Sister?" Celestia asked as she lit her fireplace. "Nothing is wrong, is there?"
Luna seemed to hesitate for a moment before replying. "I hope not, but something's been on my mind for a while now."
"A while?" With her hoof, Celestia gestured for Luna to join her near the fireplace, but Luna didn't just yet, which made the older pony a little upset.
"Since a day after Starlight left," Luna clarified.
"Haven't things been better between us?" Celestia asked, now really wanting Luna to lie next to her. "You have breakfast with me, I help you line your flowers when I'm free, and we're talking right now without passive aggressiveness. What could be wrong?"
Luna looked away for a moment, and Celestia realized in that same moment Luna's feelings mirrored her own. Luna liked things this way too, and she didn't want to ruin it.
"Come, Sister. Please lie next to me," Celestia insisted. Luna finally gave in. She sat next to Celestia, but the older pony gently pulled her down with her comforting wing. "Say it. I promise it won't ruin anything."
"I think we already are ruined, Sister."
"But... Luna... what do you mean?" Celestia asked, taken aback. "You think things are worse since Starlight left?"
"I think it's shameful we needed Starlight's help at all."
A few seconds of silence passed between the two, one worried if she should've said nothing and the other worried about how to say anything. Not knowing what else she could say, Celestia simply said, "Explain."
Luna sighed. "Sister, haven't we been through this before?"
Celestia blinked, still not catching on. A little frustrated, Luna stopped beating around the bush.
"We should know better!" she snapped, catching Celestia off-guard. "We are several centuries old! We are the last ponies who should need a lesson in talking to each other!"
"Luna, no matter how old we get, we will never be perfect," Celestia reminded her. "We will always make mistakes here and there."
"You are missing the point!" Luna argued, though her face carried an expression more of worry than anger. "We were sisters long before we were rulers. We are not perfect, of course not, but it should never be this hard merely to talk to each other! Especially after..." Luna's voice trailed off and she looked down.
"After...?"
"...Nightmare Moon," Luna finished in a whisper.
"Sister, we are past that," Celestia assured her. "Is the guilt hurting you again?"
"Do you remember what you told me when we spoke after Twilight told you about the tantabus?" Luna asked. "When you tried to blame yourself for my sin of Nightmare Moon?"
"Of course," Celestia replied, a bit somber now. "I have never thought I was faultless for the events surrounding Nightmare Moon. I swore to myself I would be a better sister when you returned because if I bothered to at least try to talk to you about how you were feeling, Nightmare Moon may have never happened."
"Can you repeat that for me?" Luna asked.
Celestia's raised an eyebrow, but did as asked. "If I bothered to at least try to talk to you---"
"Stop. That's it."
The wheels turned in Celestia's head as she tried to piece it together, and less than five seconds later, the light bulb smacked her head. "Oh..."
"Yes," Luna said with another sigh. "After all of that, we need a third party to help us learn to communicate with each other. You really don't find that shameful, Sister? I do, especially after what it cost us."
"One thousand painful years apart," Celestia remembered, wincing. "I guess I'm not keeping that promise."
"No, don't blame yourself. I also avoided speaking to you. We are both at fault for this," Luna insisted. Her voice took on an almost tearjerking tone. "But if that is not enough for us to not let this happen again, what is? Must we be apart again?"
The mere thought of losing Luna for any amount of time scared Celestia enough for her to yank Luna into her hooves and hold her tight. "Don't ever say that!" She shuddered for a moment. "Don't you dare suggest that!"
"It was a rhetorical question," Luna explained, struggling to wriggle her way out of her sister's much too tight hug. Celestia loosened her hooves enough to stop Luna from being uncomfortable, but not enough to let go of her little sister.
"I understand, Luna. You are right. For a thousand years, I never stopped being haunted by knowing all it would've taken to save you was maybe a hug, a thank-you, a chat over dinner."
"What can we do now, Sister?" Luna asked, allowing herself to sink into her sister's comfort. "What can we do to not let this happen again? I don't think we can call on Starlight every time we have a problem."
"What we've been doing," Celestia replied seriously. "What we're doing now. Making time for each other. Not saying "I'm too tired". No passive aggressiveness. You waited for me as long as you had to for us to have this discussion. This is how we prevent it, Luna. But there's never any shame in getting help, so if it comes to that, that's what we do. In the end, wouldn't you rather a third party get our heads on straight than let our troubles fester?"
"Of course!" Luna replied. "But we didn't seek Starlight. She was sent to us."
"I suppose that only makes your point clearer," Celestia said, scared for a moment to think how long they would've continued what they came to realize was a ridiculously childish way to handle things. "But I am agreeing with you, Luna. We talk. That really is all there is to it at the end of the day. We talk and we listen to each other. Like now. Aren't we doing that?"
Luna finally smiled and nodded her head. Celestia smiled too and released Luna from the hug. "Everything will be okay, dear sister," Celestia promised. "We do know better. We'll argue sometimes or do hurtful things. When you were recovering from being struck by the elements and reverted to your true self, I promised you could always come to me with anything on your mind. That hasn't changed."
"That's not one-sided," Luna told her. "I want you to speak to me too."
"Just so you know, Sister, if Nightmare Moon does happen again, I won't banish you a second time."
"But you must protect Equestria," Luna argued. "Never mind what happens to me."
"No! We just agreed that's how Nightmare Moon happened," Celestia reminded her. "Plus, you were only locked away because I used the elements improperly. They should've freed you, but one pony isn't supposed to bear all of them. If you were to transform into Nightmare Moon again, I'd break her apart."
The sheer, almost cold, seriousness in Celestia's tone startled Luna.
"I mean it. I will never let anypony or anything separate us again. If I had to enter her mind and yank you out, I'd find a way."
Despite the seriousness in her tone, Luna found that touching. "I love you, Sister. And I think you'd best get to bed."
Celestia covered her mouth to let out a huge yawn. "I love you too. Really, don't worry, Luna. What Starlight solved won't happen again. I know it, and so do you."
Luna nodded, and helped her sister into bed. The two nuzzled for a moment before Luna started to make her way to the door.
"Wait, Luna! One more thing!" Celestia called.
Luna whipped her head around, curious. "Yes?"
"...Thank you."
"You're welcome. Good night."
Luna trotted out and entered the dream realm while Celestia began to drift off beneath her covers. The night was serene, as Luna's nights so often were, and as the sun pony came closer to her own entrance into the world of dreams, she went over the conversation in her mind one last time. It was something specific Luna said that hung in her mind: "We were sisters long before we were rulers."
And that, Celestia realized, is how things needed to be.