give it a little bit more time.....more time at least to ensure that your DP is 100% happy and has some control too over what and how and where you tell her. Not least, because if and when you do tell DD1, his reaction at that very moment will stay with her, and if he is unhappy, that will have some impact.
if he is 100% committed to staying her daddy (and lets be honest here, that saying 'anyone can be a father, it takes someone special to be a daddy', is very very true), then he and she will want to still keep that bond that they already have, and you telling her just because you think she has a right to know, and not waiting until the 'really right time' IYGWIM, could have lasting consequences.
you say that DD1's biological father is still around, and yet he has made no attempt to make contact with you or his daughter....that is quite telling in itself. If you DO tell your daughter the truth, and she does want to make contact, and he rejects her, what then??? you cannot take it back, and the hurt and damage could already be done!
I am not saying don;t tell her, of course at some point she does deserve to know, but make sure it is for the right reasons, and for her benefit, and not just because you are scared of here reaction should she find out later.
I had no contact with my own father unitl I was 18, and I didn;t even find out that I had another sibling until I was 17, and yes, I was angry, but I also was old enough to handle it better and try to make my own contact, as an almost adult, rather than as a child who takes adults words much more to heart......I hope you understand what I mean??
oh, and as and when you do tell her.....make sure you have all the facts to hand, and if poss, make contact with her biological father first to make sure he is happy too......the very last thing you will need is him rejecting her at the very first point of contact.
HTH, you clearly love your family so much, it must feel very confusing to be this side of things with littles girls feelings involved.