She played family with him
If we judge by the last episode, plus the hair they found in White's room in the first episode, it can be assumed that Ivy played "family" with him all 13 years (or rather 10 years since her 16th birthday).
My only question is why did they not check her for rape/possible birth giving? It is clear that she had a baby and then lost it or something. Plus she told the Social services lady that he gave her fish just "as long as it was necessary".
All this has not been discussed at all. Or was it left for us to figure out? Thoughts?