Some good advice from other posters here. I like the idea of someone
saying something to Sis; the OP would have to decide if it was better coming directly from herself, or from her boyfriend (and of course only if BF felt the same way). It seems like the OP is close enough to Sis to give her very nice presents and has a relationship
with her and the family on her own, since BF wasn't even present at the gathering. And, IMO, this was a pretty egregious thing to do--obviously Sis may disagree that she did anything wrong, but it's not a matter of just the "tone" being hurtful or something that is open to much debate. Sis was given bracelets of a type she asked for, that were not cheap, and she gave them away to toddlers right in front of the giver, on the same day, while saying that they weren't what she wanted. If someone (like Sis) can interpret that as not
being terribly rude or hurtful, I'd be curious to hear their version...
Honestly in reality I personally would probably not ever say anything to her (or have my BF saying anything), though, because I don't like confrontation. I would just write her off in terms of gifts. I would try not to stew over it, because that only hurts me, but I would just put her in the category of people I don't get gifts for and try not to ever think about it again. I really like the person who said this was actually the "gift that keeps on giving"--one $80 gift to last a lifetime! (Because now I know not to ever give her anything again.) If she, or anyone else, makes a comment about not getting gifts for me, I would be happy to tell them why.
I also really like FoxPaws's advice:
I don't think the BF should ask for the bracelets back and I am on the fence as to whether he should say anything, but I do think that this whole incident should serve as a Yellow Flag (or maybe an orange cone?) for the OP. As in, Caution! Look carefully and watch for hazards on the road ahead.
I would be paying very close attention to the family dynamics at this point: Does everybody cater to birthday sister? Are gifts always treated as no big deal? Does BF ever confront his family? Are the children always appeased instead of being taught or told no? Is BS hostile toward the OP in other ways?
These are not questions the OP needs to answer for anyone besides herself, but I do think she needs to answer them.
Personally I don't like to make the leap to telling someone to dump their SO over one incident, but I think it's smart of them to, by themselves, figure out if this IS just one incident, or part of a larger pattern. And if part of a larger pattern, is it a pattern they can live with for the future? Patterns can be really hard to see when you're in the middle of them. Maybe the OP could ask a long-time friend if they've noticed any patterns in the things she mentions/complains about relating to BF and/or his family.