The problem with brother-sister incest is when there is issue of the union - there can be severe genetic ramifications.

Other than that, if you didn't know what you were doing was wrong, how can you blame yourself later on? If you had no reason to suspect that your partner is a blood relation, there shouldn't be guilt. Of course, being humans, we always pain ourselves about things we can't help.

The whole point of the Oedipus tragedy has been that there was nothing he could do about his fate. It's the reason I hate Greek tragedies: you pay for a sin you have committed unknowingly, and even tried to avoid.