"}In YA where the parents are still alive, we get some kind of “the authority figure has your best interests at heart scene”, which is only really justified by the auth figure admitting the kid was right this time but ruins it by going “mothers always stand by you” or something. The main character’s friend might need a new home but the main character has rightful, restricted, stability. Teachers take on the role in Harry Potter. (And you idiots’ problem is off-screen terfiness? The hell is wrong with you?)
A lot of YA and fic for adults have the bf/gf being your Main Support Person but an honest-to-god married couple is booooring so either you get miserable conflict about the relationship or the interest comes from the characters getting into a relationship. Sometimes you have a wholesome couple as supporting cast.
And that, I claim, is why a lot of works especially by women have brothers in arms stuff with actual brothers – they’re equals, with small amounts of inequality if you adjust the ages or make one disabled or something. They have Family Bonds and a reason to put up with each other and it feels natural that they’re together despite not fucking. And you’re still free to give them opposite-sex love interests with jealousy being an option for conflict but a happy resolution possible.
Like, I’ve met a pair of brothers like that and Richard Feynman was a little like this to his sister, but that shit ain’t normal, many people I know like their siblings but for all the shit I’ve got from Westerners and other non-Chinese for being an only child through no fault of my own (even ones who are fine with me being a bastard from a broken home!) they just aren’t compatible enough as people to be able to go on grand quests together.
You can have a wildly unrealistic or at least unrepresentative aristocrat family with strong ties and the characters can have a noble and conflicted lineage, but if you want to be more realistic that your main characters are nobodies, it’s not too much of a stretch to make the reader buy that two interesting powerful people cropped up in this nothingburger of a family.
Of course then if you can’t write compelling cross-sex interactions or can’t write a compelling romantic relationship, people will do the “the only well-written relationship in this work is between same-sex best friends so slash it is” now with extra incest, and people get hot for incest because it’s taboo, aaaaaand we all go to horny jail.