One of my best friends didn't find out until she was 19 that her bio dad was in prison for murder. EVERYONE in her family told her that he died. She wouldn't speak to her family for years and despite the fact that it's been 20 years still rants about how they betrayed her.

If it were me I'd want to know the truth. Maybe as your daughter matures you will be able to make a more informed decision based on her personality.