Ugh, clicked the wrong option thanks to my stupid laggy computer reloading the page on me...
Anyway, I guess I'd leave just because I really wouldn't be able to stand the thought of dating my sibling. /:
If you weren't raised with with them and you're not going to have children, then being biological siblings is just a disconcerting culturally taboo technicality. Finding out about it could make things rather awkward though.
i never had a sister before, so if it does turn out i have one, i guess i would stay with it. might be kind of platonic afterwards, but it would be nice to have a brother-sister talk, occasionally. she might even be able to hook me up with someone else, because i would prefer that to my mom doing it.
Personally, I'd end the relationship, but you know at the end of the day if a couple turn out to be siblings and stay together, it's not really anyone else's business if they are the only two concerned.
It's actually something I've heard of happening more than once but personally I just couldn't handle the creepiness of it no matter how deeply I was in love with them. I wouldn't feel guilt for what happened before I knew and would make every effort to keep them around as a sister though.