People in our NEW area don't know she's given birth to OC so I'm just trying to see how I can explain OC's presence in our family besides admitting the truth. Yes people from where we used to live before all know about OC not being mine. I just don't want to look pathetic raising a stepchild younger than our twins. Again, I did not pass OC off as a niece to friends where we used to live, her family members who know about OC, or my family. I only knew OC wasn't mine when she was born. Is that a bit clearer?
So people where you used to live know this is her child and not your child.
Your family members know that this is her child and not your child.
You neighbours and new friends will be told that she is your wife's niece; are they being told that already, since she lives with you in the new area now?
Your 5 year-old twins know that this is mummy's baby, but will be required to keep that a secret from people who will be told she is their Aunt or Uncle's baby.
Does the child call your wife "Mummy", or does she call her "Aunty"? Don't friends and neighbours already hear her saying "Mummy"? How will you explain when she is sent away and then comes back to visit? Your wife will no longer be "Mummy"; how will this happen? What is the explanation?
Your wife's brothers and sister know that this is her child and not your child, but her mother and father do not know, yet you want to ask them to raise the child.
What makes you think that they will even entertain the idea of raising the child? What will you do if they refuse?
How do you expect the 3 children to keep up this charade between the people that think the child is a niece and the ones that know she is their sister? They are only little; how much are you asking them to lie?
How do you expect the 3 children (including OC) to feel when the youngest is sent away and comes to visit in the holidays? How do you think the twins will feel when they realise that you sent their little sister away?
Are all the children going to be reprogrammed to believe that this is a niece and not a sister? Will OC be encouraged to forget that she grew up with her mother until she was a year old? If not, how do you hope to keep them from telling other people that they have a sister that lives with their granny and grandpa? When the child comes to visit, how will they taught not to tell people that she is their sister? Do you realise the levels of deception that you will be asking your young children to engage in? Do you think this is fair or right for them?
Do you think that all this deception, and sending the child away from your wife who will grieve for her, will make you able to rebuild your marriage? If you are not - all of you, including the twins who are being asked to lie, and your wife who is being asked to give up her child - happier when this whole process is over, what will the point have been?
Will you still hate OC when she comes to visit?
I think your plan is nonsense on stilts. You can't hide the fact that your wife had an OC by sending her away or pretending that she is your niece. Shed-loads of people already know from you that she is not your child, and it must be obvious to all in the new area who see your family that this is your wife's child and not yours. I can't see for one moment why your wife would agree to give up her child, and if she did, your marriage would never be happy while she mourned her, which she will.
You are not obliged to parent OC - not at all. You "hate" her, as you said in your other thread; that is your privilege. But think through all the problems and deceptions required for you to send her away, and think of the marriage you'll have left; a deeply unhappy one. You hate the child, so leave your wife to bring her up while you remain a good father to your two children.