Should I evetually tell my daughter she's adopted?

She's a baby now (11 months) but I'm debating on whether I should tell her when she's older. Her adoption was usual. My first husband turned to drugs in the last few hellish years of our marriage. He then was being unfaithful and got another woman pregnant. The mother made it clear that she didn't want the baby and was willing to sign over her rights to the child (she had at least 5 other children that had been taken away by child services for abuse and neglect). I stayed with him because I can't have biological children of my own and thought at least I can get one good thing out of this marriage. She had her and moved away and my ex had cleaned up his act for awhile but went back to drugs shortly after her birth. I left him and took the baby because he cared more about partying and drugs than his daughter. I filed for divorce and he told me that I could have her because he only wanted the baby to make me stay and then he terminated his rights after our divorce. I haven't see him in about 8 months and I have no clue where he is or what he's doing. I met my fiance about 3 weeks after I left my ex and our wedding is in 2.5 months. He's in the process of adopting her. He wants to keep the adoption a secret and make her think we're the bio parents. She's my about my shade has blue eyes and brown hair. We're both brunettes and he has blue eyes so she could definitely pass as our bio child but I don't know. My fiance says that telling her all this may make her hate herself (being willingly given up by both bio parents) but I don't feel comfortable living a lie. I don't want her to hate herself or be resentful towards us, but I think she has a right to know where she came from. Plus, the thing that's really bothering me is the other children involved. I know the bio mother had at least 5 other kids and my daughter deserves to know she has siblings out there but we can't tell her that without telling her everything. I'm so conflicted. For anyone that's ever be in a similar situation, any advice for us?