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Arcane has had me thinking about this dynamic of what I'd call "found siblings" or something like that.
The trope of "like brothers" or "like sisters" I feel is often translated wrong, or ends up becoming more romantic than inteneded is what I think is the neglect of the tie that makes siblings, siblings.
siblings are tied together by a large, fundamental simualrity that forever will tie them together, regardless of what life brigns them. Sharing the same parents intertwines two indivudals, as well as growing up in the same home, regardless of if you are related or not. This tie is a shared expeircne that shapes a persons deveolpmental years.
Also, what always makes me laugh a bit when I hear the line "they're like my brother/sister" or "We're like siblings" is the fact that most of the time, they NEVER act like siblings.
If you want to have to characters have this "found sibling" relationship, they need to be able to express anger, frustration, and a childish annoyance at the other. Siblings know that regarldess of how much they can be angry and hold a form of hatred for the other, at the end of the day, that person has been in their life so long that there are some things that that person will only understand. It's being close because this person knows too much about you, it's sometimes a resigned love. It's not talking to this person for weeks or months but still knowing every intimate and embarassing detail about how this person's tween years.
Now, I'm speaking from a biased standpoint, I'm not close with my sibling in the way in which we talk with one another every day and are best pals or something. But, they're someone I can call at the end of the day when I need help. We won't talk for a month but if I'm stuck in the ER again because of my chronic health condtion, they'll hang out with me the entire time just so i don't feel lonely.
To write characters that percive one another as siblings, they need to be characters that will always help the other out at the end of the day. They can hate one another, but still come when called. They argue, maybe they don't see each other for months because life just got in the way, but when they see one another, they go back to bickering like six year olds. Sibling dynamics are held together by the fact that they know far too much about each other to stop being friends without it being a liability.
Overall, I feel like what creates a great sibling dynamic is the resigned love both characters hold for eachother. It's an eyeroll and a sigh while reaching out a helping hand. It's disagreeing with all of their choices, but reluctantly saying yes to helping them anyway because at the end of the day, you know them too well, you've seen them at their worst and walking away just isn't an option.