What is Sansa is the younger sister, and Arya is older? What do you think would change?
Too many things to figure out any real answer. Yeah, someone might start with “oh Arya would be betrothed to Joffrey instead but she’d see right through him”, but that wouldn’t necessarily be correct, as an older-sister Arya wouldn’t have the constant inferiority complex from being compared to a more gracious older sister, plus she’d have more responsibility from being an older sister herself, and so many other factors that could lead to her being more rule-oriented and authority-following, or alternatively more visibly chafing under the yoke of the patriarchy. And consider how Catelyn having a Stark-featured child so soon after Robb would shift the family dynamics, and also affect Arya and Jon’s relationship. And an Arya approaching adolescence and looking more like how Lyanna looked before she died would also change things, especially with Ned, maybe even Robert too. You can’t just shift original-timeline Arya’s personality up a few years,
she’d be a completely different person. Same with a younger Sansa, if perhaps not quite a dramatic a personality shift.
(This is why I tend to roll my eyes a bit at fanfics where the Stark kids grow up as peasants but have the exact same personality, or if they have different parents ffs, it’s just not possible, everyone is a gestalt of their genes and experiences, and you change something huge like that, the personalities will also change hugely.)
Anyway, I can’t list changes for you, sorry. It would be a very different book, that’s all I could say for sure. Though if you still want a real answer, I suggest you ask @jimintomystery, he always has the best ideas for AUs with a dramatic shift in Arya and Sansa’s storylines like that. :)