Emotions all over the place...

i couldnt find the perfect group page for this so i just posted it on this one. I have a 6 year old. her sperm donor has never been in her life. nor is he allowed to be because I found out just what he really was after I broke up with him. he is dangerous and I was advised to not let him around my daughter. he knows she exists but said he hoped me and my "bastard child" rot in hell. so he's far away and it'll stay that way. my fiancé and I are expecting a baby girl now. he has been in my 6 year old's life since she was 9 months old. that's the only dad she knows and she absolutely adores him. she's a daddy's girl. Even when he and I weren't together for 4 years, he still got her every weekend. he still helped with school supplies and clothes, pullups etc. I never asked him to. he begged me to let him stay in her life and he begged to let him help with her financially. I allowed it. I grew up without a dad. I know what it did to me. I wasn't gonna put my little girl through that. after 4 years we got back together last year. I was diagnosed with Cervical Dysplasia, Polycystic Ovarian Syndrome, Major Depression and Hypoglycemia/Borderline Diabetic right before we got back together. he doesn't have any biological children of his own. he has always claimed my daughter as his. I have always wanted another baby. and was crushed when I was told I couldn't with all these new medical problems. I have struggled with my weight all my life as well so none of these things compliment each other. my fiance has always been VERY supportive of me and of OUR daughter. but with being told I was infertile, a month after we got back together I find out I'm pregnant. we both are over the moon ecstatic. so with all that being said, my mom and a couple other family members keep telling me I need to sit my 6 year old down and explain to her that my fiance isn't her real dad. at 6 years old she is very intelligent and I feel beyond her years. I feel like that would crush her. now, my fiance and I both agreed years ago that IF she found out we wouldn't lie to her. I actually have a picture of sperm donor put up as a "just in case" and plan to explain to her what her sperm donor was and what he did that was so inhumane I would never allow him around her if he did try. that my fiance is and always has been an amazing daddy to her and I felt not telling her was honestly protecting her. ok, just so people know, sperm donor had an ex wife. after a lot of research and calling his adoptive parents (who disowned him) and a couple police departments and detectives, we found out that he tossed his newborn baby girl in a dumpster at a hippie fest and let her die just BECAUSE she was a girl. so I guess I'm mainly after some outside opinions/advice. I have really been emotional over all this. I don't want to hurt my baby girl telling her the man she's always called daddy and who she's looked up to isn't her dad. my family seems to think she'll hate me and just say I've always lied to her if I don't tell her and she finds out some other way. so I'm struggling. the picture is my daughter last night putting a heart on her baby sister. she is so excited to be a big sister and keeps telling me and my fiance how much she loves us and how we're the best mommy and daddy in the whole universe. it just helps with the emotions to see this picture lol