Having raised two step children from toddlers, the answer is she already knows who her "father" is...
The one who loves her today, tomorrow and forever unless that love is tested by a remorseful wife..
A second more important point does her "real father" even know she is his...
What if he rejects his daughter, after all he had no problem banging his best friends wife !!!
I guess the question is genetics
Pretty low chances of that being a problem...
All in all, as they say "let sleeping dogs lay"
you are correct ...
No I disagree...you think what you want, but the child should know.