I met my older half sister only two years ago. I am my father's illegitimate son, and he confessed we were his "other" family. His confession pretty much destroyed both families, but through it all, my half sister and I met and got along really well instantly.
So well that I actually developed a crush on her. She's beautiful, unmarried, funny and incredibly intelligent (PhD and all). Despite her age she's really youthfull (she's 38, I'm 25), so we started to go out together to parties and such, and one night I ended up in her appartment, kinda drunk, and made a comment on how pretty she looked. Long story short, one thing led to another, we kissed, and we had sex. We got instantly hooked. We knew it was wrong, which is what made it so, so good.
That attraction has turned into genuine love, even if we've had to keep it secret. We've genuienely talked about finding ourselves jobs elsewhere and run away together to be a couple. I guess we'lle have tu kickstart that plan because she's pregnant. We know the risks, but we wanna keep it.