I guess I still don't really get the nephew thing. It seems important to you because you keep mentioning it. But the champagne went to your co-worker (Boss's sister). Full stop. (Unless I'm missing something.)
If they hadn't kept gabbing about it, you wouldn't have known the champagne's fate (going to her son, Boss's nephew). But it went through your co-worker first. You received the champagne and appeared to pass on it; then Boss passed on it; then Co-worker accepted it. She could have taken it home and drunk it herself, poured it down the toilet, used it for cooking, given it to someone else as a hostess gift, served it at a dinner party, sold it on eBay. It was hers to do with as she pleased. If you had a company policy that said it had to be kept at work and drunk only by employees, then she would have been breaking policy by taking it home herself. But the boss agreed that she could take it home for personal use, so whatever happens to it after that shouldn't, IMO, really factor in.
I guess I don't know what else you expected to happen to the champagne--Co-worker had a "use" for it, she just wasn't going to drink it herself. If you really wanted it for yourself, I think you should have been more clear about that when you brought it to Boss. He must have assumed/presumed that you didn't want it; otherwise he wouldn't have rendered his own judgment on it, or brought Co-worker into it. Or he understood clearly that you wanted it and is just a jerk, but that seems an unkind presumption on my part.