Wow. Yikes. That is really bad. Not only to give away brand-new gifts in full view of the giver, not only to comment that you're giving them away because they "weren't exactly what you wanted," but also to give them away to very young children who can't appreciate them and may actually be hurt by them? (Thinking of small parts/charms or clasps that pinch, for example.) That's just kind of crazy. I'm also surprised that the parents of the children didn't say something to stop it.
--way I can see it being semi-understandable would be if, as a PP said, there was a culture in the family of always being pressured to give something away to someone else if they admired it. Sister may be so used to this that she just did it automatically without even thinking, and perhaps said something about the bracelets not being what she wanted as a way to tell herself she was "okay" with her much-desired new present being given away. And she's been so brainwashed that she didn't even think to apologize/explain to the OP later.
Okay, I don't really believe that.
I think the OP would have picked up on that dynamic or at the very least her BF would have explained it when she told him what happened. Since he said not to get his sister any more presents I'm guessing he doesn't see any excuse for it. No more gifts from me.