Legal Question in Family Law in Illinois

Even though he gave up his parental rights (which I find very difficult to believe in a divorce, those rights were terminated in the adoption. However they are still half sisters. That gives her no rights, other than a sibling desire to meet, The ex has no rights, assuming that the adoption was proper.

Your adopted daughter is now an adult regardless of the status of bio-Dad's parental rights. The adoption terminated any residual rights which he may have had after the divorce assuming, of course, that the adoption was proper. Even if it wasn't, he's not attempting to exercise any parental rights over an adult child, his daughter is simply trying to locate a biologically related sibling. I don't believe, based on adoption laws, that the two girls are legally "half-sisters" but they are biologically. Unless the 16 year old is making financial or other demands, you should leave it up to your daughter as to whether or not the girls meet. Your daughter may also want to meet her bio-Dad which is something you cannot stop given her age. I think you have to back out and let the two girls firgure it out for themselves.