You've already had lots of great responses OP. Your friend definitely needs to know how you feel. I'm so sorry this has happened to you.
Re your DD and DS, my own story might be of some use/hope. I was conceived via a process similar to IVF back in the age when this was unusual/new. My parents didn't tell me. I found out when I was in my teens, from a family friend's DD who had known forever. (The parallels to your story are quite similar!)
It was the sort of thing everyone knew – grandparents, family friends, aunts and uncles. Everyone just thought "Oh let's not bother telling DoggRadio1."
Bizarrely, the whole thing ended up making me so much closer to my parents. They sat me down and talked about it and apologised. They showed some vulnerability by apologising for their mistake and I showed some vulnerability through my shock and confusion. We got through it by being honest: I asked questions, they answered, we showed that we are human beings with the capacity for error, forgiveness.
It took a while to recover but we got there in the end, and many years on we are now a loving, supporting and trusting family. (I am an independent adult now but we are all very close.) You will get there. Big hugs x