It can be pretty hard sometimes, I’ll tell you. (BTW, during the BotBW Sansa is 12 and Sandor is 28.) Generally I deal with it by only romantically shipping in the potential future of the story, and in romantic fanfics I prefer futurefics (assuming a time gap in which Sansa reaches adulthood) or aged up fics. (Aging up sometimes bothers me, especially if it just seems like an afterthought with no actual effect on the story, but I have seen it done well in stories where everyone is aged up 5-6 years and furthermore the characters are handled appropriately. A 16-year-old Sansa would not have been quite so innocent and naive as the 11-year-old fooled by Cersei and Joffrey.)
However, the greater problem is not in fanfic, it’s in the books themselves. Initially GRRM planned for ASOIAF to take place over the span of many years, maybe a decade, and started the kids out young because he wanted them to reach adulthood for endgame. AGOT itself was supposed to take place over years, and it was only when GRRM was writing it that he realized that events moved too quickly and only a year or so had passed. So he resolved this by planning a 5 year gap that would take place after ASOS (which is why that book resolves so many plot points) – and when the storyline started up again, all the characters would be 5 years older. So for example, by endgame Sansa would be at least 18 to Sandor’s 34. (I’m not sure if 18-34 would have been all right for you, but note it’s almost the exact same age difference as Jaime and Brienne.) However, GRRM couldn’t make the timejump work, and therefore scrapped it. (One of the reasons AFFC took so long to be finished.) Honestly, once he noticed the problem in AGOT he should have aged up everyone right there before publishing it… but oh well.
Anyway, so the bigger problem is that even with the lack of the time gap GRRM still hasn’t changed his plans with how he’s handling the kids. So you end up with things like Arya’s sexualization in TWOW’s Mercy, and Sansa’s flirtatiousness in her TWOW preview chapter is probably only the start with her. (But please note the “controversy” was not what was suspected.) So if GRRM does actually do romantic sansan in the books (and though I can’t be certain of it I don’t rule it out either) it may be more “unsettling” there than most conscientious fanfic authors would write. Actually, cancel that “may” – I’m sure that whatever GRRM writes (sansan or otherwise) will be more unsettling than most fics. (Though not all of them *cough* GOT *cough*)
And there’s also the even greater problem with how GRRM normalizes sexual matters happening to 12-14-year-olds even while saying it’s abnormal for Westeros, and other gender issues… but that’s beyond the scope of my reply. Although it does result in people rationalizing sansan with “Sansa was married at 13 and expected to have sex” or pointing to Dany & Drogo and saying “you didn’t complain about that!” (some people did complain, even if hypocrites exist too) and y’know… just because GRRM made it “normal” in ASOIAF still doesn’t make it right, or make it look good to most people. The false pedophilia accusations should be dealt with of course, but with any other argument other than “it’s normal in the books”, please.
Just… Sansa and Sandor had a strong emotional connection in King’s Landing that did involve some sexual tension, and it was kind of fucked up, and we shippers should admit it was fucked up, we’re aware of it and have to deal with it, and we hope for a happy healthy relationship that Sansa chooses as an adult. (And if someone reading this is a shipper who doesn’t ship it that way… well, you do you, I’m not trying to speak for you, but… I avoid your fics if I can help it, sorry.) And in the books themselves, if we don’t get that kind of future relationship, well, hopefully whatever we do get will not be too unsettling, and hopefully mostly satisfying. For some fans at least. Whoever they ship.
(BTW I haven’t stated this in my author’s notes yet, but my own sansan fic is incorporating the five-year-gap that never happened,
although not that big a leap (more like 3 years) because I can’t really
make it work with the books’ storyline any better than GRRM could. It’s hedging, I
know, I’m mostly dealing with it by pretending ASOS/AFFC/ADWD took a
lot longer than they actually did and also by not mentioning timeline
issues if I can help it. Sigh, oh GRRM. Oh, me.)