Definitely don’t tell any lies. When she is a teenager she’s going to do a DNA test on Ancestry.com or similar, and she will find some kind of distant relations on there, and might even track down her dad. So don’t give her any information now that teenage her could be upset about - or any dangerous illusions about searching for a kind dad who doesn’t know about her, either.
Kids understand blunt facts but get confused by evasion. I would probably say something like…
“To make a baby, a man and a woman need to have sex. The sex takes what we call genes from the man, and also some genes from the woman, and mixes them together to make a new person. Genes are basically just instructions for the woman’s body on how to grow a baby. A bit like how the instructions in a lego set tell you how to make the lego.
Usually when that happens, the man and woman have been very good friends for a long time, and have chosen to make a baby together, and they both look after it together. Or, sometimes after a while they want to live seperately, and so the baby only sees the dad sometimes, like with your sister.
What happened with you was a bit different. I was feeling lonely and I was in a confused time of my life where I really needed some grown up cuddles, and so I made friends with a few different men, just for a little bit, and I had sex with them. We weren’t trying to make a baby, sometimes grown ups have sex just for the cuddles. Later I stopped seeing those men, because it turned out they weren’t so nice to me, they could even be quite mean and unfriendly to be honest. Have you ever met someone you thought would be nice, but then later they were not nice to you? It was like that.
Later, I found out that my body was growing a baby, and I was so happy and excited about you! But, I didn’t know which man had given my body the genes to start the baby. And I didn’t know where any of those men had gone, I didn’t even have their phone numbers. And I thought oh well maybe that is better anyway, because those men weren’t very nice to me, so they probably wouldn’t be so nice to a baby either. I love you so much and I wanted to be sure you only have good kind people looking after you. And although I didn’t know which man gave me the genes to make you, I knew he was one of the not so nice men. I didn’t want you to have to go and stay with any of those men. So that’s what happened. Is there anything you’d like to ask me about that, or shall I show you a youtube video of how the genes from the man meet the genes of a woman and give her body instructions on how to make a baby?” Then distract her with one of the kid-friendly cartoons of sperm meeting egg.