Someone has probably made a meta out there about this, but I haven’t seen it yet so I figured I’d write up what I saw… Sansa only thinks of Jon 4 times in her entire narrative (five if the second one counts as two). However, she increasingly begins to soften her descriptions and thoughts on him as she grows up.
In AGOT Sansa I, she not only focuses on his bastard status, but calls him half brother, cementing the differences:
It would have been easier if Arya had been a bastard, like their half brother Jon.
This continues in AGOT Sansa III, but is associated with slightly more gentle thoughts than the last mention, as she pities him instead of thinking on the differences:
But this man had been crookbacked and hideous, and he looked as though he might have lice. If this was what the Night’s Watch was truly like, she felt sorry for her bastard half brother, Jon.
I would think it is rather against her previous thoughts to not use the entire epithet she’s always used. While ACOK only has a single mention, in Sansa V, she drops the half.
She sang for mercy, for the living and the dead alike, for Bran and Rickon and Robb, for her sister Arya and her bastard brother Jon Snow, away off on the Wall.
In AGOT Jon III, he even points out that this is how she always refers to him:
He missed the girls too, even Sansa, who never called him anything but “my half brother” since she was old enough to understand what bastard meant.
So calling him “brother” reflects those other changes people have discussed at length- she cares less for heroes and valiance and honor, because those don’t exist. The tinge of shame of a bastard is less prevalent in her life, now.
Finally, in AFFC Alayne II, she acknowledges that he is “only” half a brother, but does not think that him being a bastard is a vulgar thing anymore, especially since they have a shared connection of their bastardry:
…Oh, and the Night’s Watch has a boy commander, some bastard son of Eddard Stark’s.”
“Jon Snow?” she blurted out, surprised.
“Snow? Yes, it would be Snow, I suppose.”
She had not thought of Jon in ages. He was only her half brother, but still … with Robb and Bran and Rickon dead, Jon Snow was the only brother that remained to her. I am a bastard too now, just like him. Oh, it would be so sweet, to see him once again. But of course that could never be. Alayne Stone had no brothers, baseborn or otherwise.
Her experiences cause Sansa to soften towards the world of propriety and decorum, and reflects just how changed she has become.