Things are going extraordinarily well, should you think there might be problems. Our children understand about their "super cousin" with our oldest child's concern that he is still the oldest of everyone. Old is Big for kids.
Even more crazy, I think the super cousin looks more like our older child than the twins.
And craziest, we are nothing but excited that we could help our friend start and finish her family. This is not our children's sibling. This is a super cousin. More than a cousin, but not a sibling.
I don't think that everyone can do what we have done. One of my strengths is the ability to draw and maintain very strong boundaries. I fully understand how adopting a FET to another family could be hard and confusing. But it doesn't have to be. And it can be quite fulfilling with some substantial positive emotions.
We are thrilled for our friend--even the children who pray for her and her child every night. They confuse Baby ***** with Baby Jesus, but that is a whole other story.