If Tea was not Irina's daughter...(SPOILER ALERT!!)
...then isn't it a overly great coincidence that the family, to which's life Irina enters as a maid in order to find her child, also has an adopted child. Considering that the family name, which is given to Irina by Lucrezia, was inaccurate, I think it is a little contrived part in the story that Adachers have also an adopted child.
I really wonder the explanations for this!