So do they accept that since the child is yours, they will either have to come out to their family or you will keep the actual paternity of your child from these members of the family?
And since she is infertile and 46, no one will be fooled that she is the biological mother, especially if you are out with them...what kind of crazy looks will you get with a child who might clearly look like you and the husband with this other woman claiming the title 'Mummy'? It will totally mess with this kids head too.
Be mummy, if she wants to be Ema, Tante, Aunty or any other title that is fine, but you are the Mother! Everyone and everyone' Uncle Bob better know that and there should not be any care about them taking it 'really well' that is a bloomin fact!!!
I still think you are being hopelessly naive about the effect motherhood will have on you. Trust me, if she tries to claim maternity over this baby you will be near homicidal. I say that as the mother of an infant.