I'd be as open as you can be.

From the other side - the birth family:
We had the family nearly torn apart with an adopted child finding their birth mother and the rest of us never knew that she had given up a child for adoption, so it was just dumped on everybody and it caused no end of heartache, not only with the birth family (mine) but the adopted family (they were unaware their child was trying to find their birth parents, but would have supported if they knew), the adopted child decided that they wanted to be a part of the family and almost broke up a marriage over it.