One thing I noticed in some of the Sansa illustrations I've seen is that she's made to look much older than Arya, when she's only ahead by 2 years. They're still considered little girls (by modern standards) at the start of the series, as shown here. My source of inspiration for this is Judy Blume's 'The Pain and the Great One',
a really cute children's picture book I have… I don't see Sansa and Arya's relationship as antagonistic (at least, prior to the Joffrey incident) as the siblings there, but after all that's happened, I’m not sure how it will be when and if they reunite at the end of the books.
Credit goes to env1ro
for the paper texture I used.