It's time to tell my daughter who her bio parents are, but how?

Hi there, I'm new here but I've read quite a few posts and they've helped a ton. Not all have answered all of my questions so I thought I'd just ask my own. A little back story, my husband and I were married for 5 year and had tried every route of fertility treatment in our means. My sister in law text my husband and asked to barrow money. (She did not know yet the doctor had just 3 days prior confirmed there was no physical way I would ever conceive on my own or carry a baby past 12 weeks.) He asked her for what and she said for an abortion. She had 2 other kids that she was barely able to take care of and knew she couldn't add another with the father abandoning her the second she told him she was pregnant. We didn't reply right away as that was very painful for us to even imagine helping with just being given the news from the doctor. A day or 2 later, she calls and says she had talked to my mother in law and was wondering if we'd like to adopt the baby and have her and her kids live with us so we could be involved through the whole pregnancy and help her as well. Long story short we did and everything was amazing. Fast forward 5 years we told my daughter she was adopted but were suggested because of being a relative to wait until she asked to tell her who her bio parents were. So here we are to the reason for me joining and my post. My daughter is now 12, and because i.f a school project in genetics the question had arrived. We've for the last 5 years been estranged from my sister in law because of some major drug and lifestyle choices we are not supportive of and choose to be distant from. We're still in contact with my niece and nephew and to her, her cousins. My sister in law has recently told my niece and nephew that my daughter is their bio-sister and at their next chance to tell my daughter so she'll come running back to her and leave us because I'd the horrible people we are. We know that wont be the issue because with my daughter seeing how she lives and the choices she's made, she dispises her. We are planning to tell my daughter in ther next day or 2 because my niece wants a sleep over again and don't want this to be taken out on my niece but I want our daughter to hear and feel the news of her bio from us not her cousin. So the question, how do I start the conversation? I don't want to just: "Hey, the moon is huge tonight, by the way meme is your bio mom." How do I work into it? Also, we know who the bio- father is but have no contact but he's very active on Facebook, (because I check his page often) so I believe there could be contact if she wanted. There is also more sibling on that side as well. What reaction should I prepare for from her as well? I know everyone is different but a little insight would help my mind prepare as well. Please, any input would be very appreciated. Thank you so much.