I fully understand the level of guilt, shame, humiliation, etc. behind the "MYOB" answer, but here's the thing: Shame thrives in darkness, and the world is a (slightly) different place than it was 50 years ago. Either way, mom is now dead, so there isn't someone to upset on this side of the family. Maybe dad knew about the pregnancy and didn't want to stick around, maybe dad lied to mom to get laid and thus wasn't going to commit, maybe dad has really good and valid reasons for why he couldn't commit, maybe dad has no idea and would be thrilled to find out he has a kid, or maybe dead is now dead, so there isn't someone to upset on that side of the family, either.

Daughter deserves the option of closure and the option to know where she comes from.