Half-siblings... it's complicated

My sister was faced with the same situation as you when she adopted her daughter about 12 years ago. My niece was 17 months old when my sister got her as a foster child and about 3 when the adoption was finalized. During the 1.5 years prior to finalization she had ongoing visits with her older brother who was 11 when they were brought into care and had just started visits with her newborn brother. My sister had intended to allow these visits to continue after the adoption was finalized until she learned that the newborn would be returning to bio-mom (in a drug treatment center) and the older brother would be remaining in fostercare with bio-mom visits and a goal of reunification. She did not want her daughter to have contact with the brothers if they were going to remain in contact with bio-mom because she thought it would be difficult for her to understand how come they could see bio-mom (and maybe even live with bio-mom) and she couldn't. Also, quite simply, she didn't want her daughter having that close of a connection to someone who she believed was a bad influence for her. Cutting off contact with the older brother was a particularly difficult decision for my sister being that he had practically raised my niece for the first 17 months of her life, but she felt that it was the best decision for her daughter. All I can tell you is that today my niece is one of the most well-adjusted 14-year olds I have ever met. She seems to have suffered no ill effects from losing contact with her brothers. She knows that they exist and she knows why she can't see them, but she doesn't seem to have a problem with it all. If not having visitation with the brother is important to you, then I would recommend checking to see what your legal rights are. I'm in the process of being licensed for fost/adopt and they told us in the classes that once the adoption is finalized that sibling visits would be solely our choice (although they highly recommend it). I don't know if the laws are the same everywhere, though.