I recommend making any future gift giving (or not) the BF's responsibility. If he gets a gift for his sister, it's from him and the OP. If sister complains about a gift or lack thereof, she can be told to direct her complaints to the BF.
Without further information, I'm not willing to criticize the parents of the girls who received the bracelets. They were at the party with three young children, and I imagine most of their attention was on the children. They may not have been aware of the significance or cost of the bracelets. At the time the bracelets were handed over, the parents were trying to wrangle three children out the door. The regifting may have been barely a blip in the chaos. It may have been like "Oh thanks . . . Susie, don't take your coat off . . . Tiffany, put the cat down . . . Brian, you can have a drink when we get in the car . . . Susie, I said don't take your coat off . . ."