Ran likes playing house with Onii-chan and Haru-chan, because she's happy that they always do what she tells them to. Except sometimes Onii-chan and Haru-chan aren't being serious enough, and they always...
"C–Chew, chew, chew… I ate it."
"Pffftt! That's a rare sight coming from Haru."
"...Dogs don't talk."
"Don't be mean, woof~"
"Geez! Onii-chan and Haru-chan, stop flirting with each other every time!"
As soon as she snaps, the room turns silent. She looks at Makoto. His mouth is half open and she wonders why he's turning pink. Then she turns to look at Haru.
He just stares at her blankly. She blinks.
"Onii-chan and Haru-chan were 'flirting', right?"
Makoto gives a cough before responding, his voice coming out as a squeak.
"Ran! Where did you learn that term!"
"Huh?" Ran looks at him, bewildered. She places a hand to her chin. "The other day, on TV... There was a Papa and a Mama who were being... lovey-dovey..." Ran tries her best to recall...
"Ehe~! That's pretty rare coming from you, dear~ You're embarrassed, how cute!"
"I'm not... You always talk too much."
"You're always so mean~"
"And then that girl said-"
"Geez! Papa and Mama, stop flirting with each other already! It's so embarrassing!"
"...Onii-chan and Haru-chan sounded just like them, so I told you to 'stop flirting!' " She looks between them. "Is that wrong?"
Haru looks on, expressionless, but he can feel his cheeks rapidly heating up. Makoto looks as red as a tomato now, and he glances at Haru apologetically.
He clears his throat for the second time. "Ran, that's—"
"Ehhhhhh? If Haru-chan and Onii-chan are Papa and Mama, does that mean that Haru-chan is going to marry Onii-chan? No fair, I want to marry Onii-chan too!" ...Now even Ren is joining in!
Ran's eyes widen in realization. She pauses to think, and nods to herself in approval. She wants to marry Makoto too, but if this is the case, then it can't be helped.
Makoto is now too embarrassed to say anything, while Haru just waits as Ran approaches him.
"Haru-chan..." Ran places a hand on his shoulder and looks at him with a serious expression on her face. "Please make Onii-chan happy."
Haru can't help but laugh inside. He simply deadpans, "I will."
Ren pouts. "I knew it. We have no chance against Haru-chan..."
Makoto feels like burying his face in a pillow. They're just children, but really, what are his siblings even saying!? He looks at Haru, expecting him to say something to help him, but he regrets it immediately when he recognizes the mischievous glint in his eyes.
"That's good, right? Makoto."
But when Makoto notices the corners of Haru's lips twitch upwards ever so slightly, he can't help but smile himself. He sighs in resignation.
"...Let's have a wedding ceremony for Onii-chan and Haru-chan!"
"Haru, you don't have to play along with her if you don't want to..."
"No... I'll do it."
"Haru-chan, will you take Onii-chan as your wife...Husband? Do you commit yourself to his happiness and uhh... self-fulfillment as a person? Do you promise to love, honor, and trust him in sickness and in health, in a-adversity and prosperity, and to be true and loyal to him so long as you both shall live?" Ran looks proud of herself as she finishes reciting the wedding vow from the script they had prepared for the ceremony.
"I do." Haru says simply.
"Haru-chan, you have to look at Onii-chan when you say that!"
Ocean blue eyes meet forest green ones. It feels embarrassing somehow.
"...I do." He repeats as he tries to shift his gaze away.
"What about you, Onii-chan?"
"E-Eh?" Makoto finds himself distracted by a blushing Haru.
"Y-Yes. I... I want to be with Haru... forever."
"You may now exchange your rings!"
Ren presents the rings that they had made using paper, and the two exchanged them. Makoto finds himself blushing when Haru holds his hand. Even though they have been holding each other's hands ever since they were little...
"Haru-chan, you may now kiss the bride!"
Makoto is snapped out of his train of thoughts immediately. "What!?"
Haru leans in close.
"H-Haru, you know don't have to..."
In front of the kids? Are you sure? Wait, I'm the bride!?
When he looks into Haru's eyes, he realizes Haru is serious.
"Makoto..." I'm sure.
He feels a pair of soft lips press against his. It's a chaste kiss; but Makoto feels his face heat up considerably.
Ran squeals in delight while Ren hides his face in his hands.
In the end they're playing house again because Ran insists that it's not fair since it was interrupted before. This time, Haru plays the dad and Makoto plays the mom.
"Welcome back, Haru. Ran, Ren, greet Daddy."
"Papa!" Ran and Ren run forward to hug Haru.
Makoto can't help but smile at the scene.
"...Makoto, what's for dinner today?" Haru looks at him expectantly.
You should know. "It's your favorite, mackerel."
The next morning...
"Ne ne, Okaa-san, Onii-chan and Haru-chan got married yesterday!" Ren exclaims excitedly.
Makoto almost chokes on his hotcakes.
"Is that so?" Mrs Tachibana chuckles good-naturedly.
"Haru-chan even kissed Onii-chan!" Ran contributes, causing him to blush hotly.
"Oh?" Now even his father has put down his newspapers and expressed interest in the conversation.
His parents look at each other and smile.
"Makoto," His mother addresses him with the knowing smile she always wears. "Make sure to take even better care of Haruka-kun from now on and make him happy, alright?"
Makoto is about to turn to Haru for help when he feels Haru nudging him from under the table.
Haru? He looks at Haru, questioning.
That's good, right? Makoto.
A/N: The prompt itself is adorable, so I had fun writing this! It's short, but I hope you enjoyed it :) I love domestic MakoHaru! Headcanon: The entire Tachibana household ships MakoHaru (lol)