She doesnt remeber her bio dad I think was too young!

My mum has kept a photo of her bio dad for when the time came, which i guess is lucky, I can get a pic off FB though, we have 14 mutual friends considering we went to school together!

What do I tell her and what do i leave out, ill share a quick story you know where its all coning from

I met X in high school when I was 16 we got together and when I was 17 I fell pregnant with DD, X decided the baby wasnt his and refused to have anything to do with me, he orderd DNA testing which of course came back positive, he was in and out of her life until she was 14 months when I met DH and moved 4 hours away, I then got summoned to court and he had fortnightly supervised access this went on for 5 months when she 26 months old he decided with his uni & work he didnt have time to travel to see her so stood back and hasnt seen her since.
He now has another child, emily, shes 2 now do we tell DD1 about her sister? Hes now an architect in Melbourne and his GF is a Lawyer! (whom I hope isnt a BB member, if you are then im sorry & please speak up)