that their big sister has a different biological father. I split with DD1's dad when she was a few months old. He was v abusive to me and her, and was not allowed any contact whatsoever, or any form of PR. DD1 is now 12 and we haven't heard anything from or of him for nearly 12 years. DD1 knows all about it - when she was younger I used to talk about her 'father' to her because obviously she has a right to know. But over the years it's been talked about less and less. I was always very open with her about why there was never any contact, which was hard to do but I felt I had to be completely honest about it, in as gentle way as I could be.
When DD1 was three I remarried to my now DH. We had two more children and DH is now DD1s legal parent as well as her dad 100% in her eyes.
So it may sound strange, but I am shocked because it's something I almost never think about, and is not an issue to me, or to my eldest daughter, so it's suddenly hit me that, as it's something way in the past now, we never talk about it within our family. Therefore, DD1's younger siblings (7 &3) don't know anything about it. Therefore, it also feels to me now that I've been inadvertently keeping something from them - and I don't know how to broach the subject now 
I don't want to have a big family secret from my younger children, but I don't have a clue what to say to them. How would they even understand what it means to have a different (biological) dad when they don't understand about sex yet? But if I don't start mentioning it soon, surely it will then turn into a big secret which they could stumble upon one day when theyre older and then feel quite shocked?
It probably sounds horrible too, but I also now feel that we are a happy and 'real' family, and by telling them the technicalities of their elder sister being actually their half sister, I could make it all feel different, for them and for her. I almost feel ashamed of my past now because everything is so different now. It's as if I imagine my younger children see me as this lovely happy mummy who does everything right, and I've now got to tell them that actually I used to be a huge fuck up. Can't believe how hard this realisation has hit me TBH 