Saraswati was not born from Bramha's relationship with any other woman. She was created by Bramha just the way he created everything else. I mean if he created Himalaya and if he created Shiva then isn't Shiva, Parvati's uncle? 

Incest is a very modern concept probably derived from practical experience than inbreeding leads to defects. I dont think it can be applied in a mythological structure like this.