Honestly, they've established that she's long-lived enough that time doesn't feel as long as it does for the average person. That doesn't make it any less weird, but I'm comforting myself with that until I can either convince myself to keep going or drop it once the romance hits.
I've dropped something with those hinky parental incest vibes before — it's OK to enjoy the childhood development and then dip out once it hits the point where it'll start to ruin everything that came before. Since this is a Korean title, I'd dip out as soon as Tita turns 18 or older if that's a serious concern for you. Or cut your losses now if you prefer.