I believe, the 30 seconds it took 11-year-old Sirius to say,
"Maybe I'll break the tradition,"
has more depth than all the Slytherin characters combined. Do you guys really understand how much courage it must have taken him to say this? People love to portray this scene as, "Oh, Sirius was obsessed with James and he chose whatever would keep them together," but... I simply don't think so.
Sirius grew up in a cult, surrounded by an insane amount of propaganda and his family's emotional projection since he was the heir. (And he was probably the perfect fit for the heir, too. I like to think Orion and Walburga saw his bravery, charm, audacity, defiance and arrogance as a "strong personality" and high-fived each other because their firstborn wasn't some passive, soft-headed bloke (like Regulus) who couldn't properly represent the Noble House of Black. I know Sirius probably resisted their wants and argued with them even pre-Hogwarts, meanwhile Regulus did everything they wanted and the most prominent thing about him in those years was his submission—although I don't think they even once wished for Regulus to be their firstborn. They liked Regulus's behavior, but they wanted Sirius to embody it.)
I also like to headcanon that Walburga never once truly believed Sirius betrayed the Potters. Because yeah, he was her firstborn—her fierce son with a sharp personality, the perfect fit for the Black heir. She knew her baby, and probably loved him too, just improperly. I know this fandom loves to make her an evil, overly sadistic parent to the point that it leans into misery porn, but I like to think of Sirius's relationship with his parents as something much more emotionally destructive.
I personally think the fact that she never once disowned him properly is enough evidence stand-alone. I mean, why did they let Sirius take all of that fortune—and even let him leave it to whoever he wanted (as Harry got it at the end)?
(Because disowning Sirius completely and legally would mean admitting defeat. It would mean admitting that they completely lost control of him. Instead, they kept him on the family registry and left the fortune intact. So it felt less like an amputation of a limp for them and more like a bad decision their teenage son made.)
Do people know how much courage it takes to even think about doing something outside of the standards when you grew up in a closed environment like that? It took Draco until the end of the seventh book to do something—to lie to his aunt and tell her that it wasn't Harry Potter.
An eleven-year-old is a child, a middle schooler, roughly even conscious. I like to think his House sorting was something he thought a lot about beforehand. (Maybe he even decided to follow the first friend he made to their House, so it was really all because of James, but not about James. Or maybe he decided to walk up to that Hat mumbling to himself, "Anything but Slytherin," just like Harry did. Or maybe it was a total shock for him like everybody else, though I don't think so.) It was something he thought and stressed about.
I like to headcanon that Andromeda married Ted Tonks just the summer before Sirius's first year, so it was like a double whiplash for the family, you see? It was also what gave Sirius the daring to even hope for another way than being a Slytherin.
Why? Because in cults, the conformity relies on the illusion that no alternative exists. The entire illusion shattered for everyone that summer. Andromeda, by marrying a muggle-born, provided a cognitive opening- a containment breach for the whole politic superiority complex Blacks had built their noble bloodline onto.
And well, when he finally did get sorted into Gryffindor, he immediately sat next to Lily (a muggle-born) canonically—remember? Do you see the immediate difference in the path he would take for the rest of his life? Sirius getting sorted into Gryffindor was one of the most powerful rebellions in the whole franchise, purely because of the emotional labor it put 11-year-old Sirius through.
It was hard. It was hard. Seeing through the propaganda at 11 is hard. Realizing maybe your parents are not that great is hard. Accepting it is somehow even harder. I like to think Walburga was the kind of mother who shows affection— but only if her sons are "good enough." She'll hug them, make them sit on her lap, and kiss their faces, but only after they do something that aligns with her values, proving to her that her two children are taking the path she wants them to take. I don't really know what practices those could have been, but I know that offering affection as a rewards for compliance was 100% more effective than fucking Cruciating them constantly (I honestly hate this headcanon with a passion, do people even know how systematic emotional oppression in abusive families work?)
It's harder to leave your family if you could just not speak, and suddenly your mother is waking you up by caressing your hair and your father is asking about your day at the dinner table. There is a saying,
A baby cannot hate a mother without the mother first hating the baby,
and I agree. But if you were made to believe that you could simply prevent your mother's hate by being a good boy, wouldn't you have tried to do that? It is a survival instinct for a child to try to comply first before completely rebel. Wouldn't you try to fit yourself into her standards at first, work hard for your mother's love to actually earn it, and then look at the ceiling of your room at night trying to decide whether you could live like this forever or not?
If love is a form of transaction which uses your autonomy as it's currecy, how much could you really afford? Or how much would you be eager to sacrifice to afford more?
Even if it wasn't ultimately his decision and the Hat just wanted to put him in Gryffindor, the fact that 11-year-old Sirius Black didn't desperately beg the Sorting Hat to put him in Slytherin instead is one of the strongest manifestations of resistance I've ever seen in media.