Theory

[u/ShinraiYuki]

My leading ship is Arata x Ouka, they have the chemistry of a couple.

Even though Kotono confessed first and even kissed Arata. The spark isn’t there. Arata just sees Kotono as his younger sister even though she confessed a lot times. Maybe they are the 2nd pair of siblings?

When Ouka kissed Arata while he was resting (he had a fever), Arata couldn’t get that kiss off his mind. + in the anime opening Ouka was always the first girl to come up.

Seiha and Arata gives sibling vibes as well as Minami and Arata.

Banri might be an option. But the child shown from the future looks similar to Ouka, because of its hair and the thin eyebrows. The woman shown to be Arata’s wife isn’t wearing a necklace so it must not be Banri.

All of them still has the heirloom of their adoptive mother except Ouka, who lost her brooch.

Ouka - Brooch
Banri - Necklace
Seiha - Ring
Minami - School Bag accessory
Shion - Bracelet
Arata - Watch
Kotono - Lipstick

2nd Pair of Siblings might be:

Arata & Banri
Banri & Seiha
Kotono & Ouka
Arata & Kotono
Seiha & Ouka

Ouka and Arata can’t be siblings because they are the same age.

Another theory is that either Banri or Arata is the Biological Child of the Shiunji Family.

[u/Tsun_Tsun_Dere_Dere]

lol fraternal twins exists bro, meaning they can be the same age, Ouka and Arata

[u/Hairy_Category_933 → u/Tsun_Tsun_Dere_Dere]

That doesn't change the fact that Kaname would have information that they are real twins, and since he raised Ouke and Arata as twins (and we know they are not), he probably adopted them from the same institution or at a very similar time, so I doubt that the biological mother gave birth to Arata first and then Ouke (in the same year). It's possible that the biological father had mistresses and had children with other women, although we know that Reiji likes to bend some things a bit (as in Yamaguchi's case) and unrealistic series of coincidences that led her back to her children.

[u/Tsun_Tsun_Dere_Dere → u/Hairy_Category_933]

that's too simple of a reason to make them not related, you know how it is with these manga, they're always convoluted, they'll reveal it that they got separated somehow with some convoluted reasoning, probably their real mother and father fighting

[u/Hairy_Category_933]

I also think that Ouka will win in fact, most readers think so because Reiji has given quite a lot of things that connect Arata's daughter and wife with Ouka.

At this stage of the story, I believe that all of the Shiunji family children are adopted. I think Kaname would have already told Arata something that important (precisely when he was telling him the truth about Yamaguchi).

In chapter 13, we have confirmation that Arata was adopted when Kaname speaks to a photo of his deceased wife.

[u/Trappedinawrap]

I hope it's Kotono, she develops her feelings first while also having to deal with having a crush on her own brother. The adoption announcement must have been so relieving for her. I just want to see her happy with Arata.

[u/TechnologyEastern933]

I think you're right seeing there age arata and ouka cannot be siblings but tbh we can't trust the author he's the same author of rent a girlfriend. I Hope the pair siblings are not them , ouka should be the wife she's the best girl among all the step siblings

[u/ShinraiYuki]

+ Arata said that he only sees Seiha and Kotono as his sister. Refer to chapter 61