Both of my parents were previously married and my brother and sister were the product of my mum's first marriage. I am the product of my mum's second and my dad's third marriages (dad's now onto wife number four!). My mum and dad got together when my sister was one and my brother two years old. I was born a few years later.

We were never told that dad was not my brother and sister's biological father. When I was turning 14 I went hunting for my birthday presents (as you do!) and I found some official paperwork relating to changing my brother and sisters' surname. It wasn't too hard to put it all together (they look very alike and I look very different, they had always teased me that I was adopted!). I confronted my mum with what I knew a few days later. She was shocked and unprepared and initially denied that it was true. A couple of days later mum and dad sat us all down and told us the truth.

My brother had figured out the truth a few years earlier but had never said anything for fear of hurting dad. My sister was utterly shocked and found it very difficult to cope. Even now, almost 20 years late, my sister feels betrayed and abandoned. Betrayed that our dad had 'deceived' her and abandoned by her biological father (whom she has contacted by doesn't maintain a relationship with).

Having lived this situation and seen the effect it has had, particularly on my sister, my advice to you is to tell your DD now. Don't wait. You and your DH need to sit down together and answer all her questions honestly. Reassure her that her dad is still her dad and loves her just as much as if she carried her DNA. She needs to feel confident that her dad loves her as much as he loves his biological children. A dad is someone who raises a child, not someone who contributes some genes and walks away.

I would not tell her yet of her half sister unless she asks. It was one of the first questions my siblings and I wanted answered - are there more of us? But we were older and your DD may not ask yet. When she does ask, you must be honest. If she hasn't asked in the next few months I would find a time to tell her, it's important that she knows you have been honest and told her the whole truth.

Good luck. Oh, and for your DH - both my brother and sister consider my dad to be just as much their dad. He is the man who raised them, supported them and loved them. A family is a lot more than genetics.