Pretty OOC I'm afraid! Viola simply isn't the kind to go for incest; she tends to take those taboos fairly seriously, or at least seriously enough not to transgress them herself. However, if you wanted to think up a fanon scenario, I'd recommend two factors: intoxication, and Alec manipulation. Alec has treated grooming Viola into a threeway with Isabelle as a long-term project for years, even if he doesn't canonically succeed.