I agree that there isn't much you can say at this point. Perhaps in the future if you neglect to mail a graduation card or something and she calls you on it then you can remind her of this, but then it comes down to if you are willing to punish her kids for her behaviour.
Personally, I grew up nearly the youngest of one set of cousins, and by far the oldest on the other side. There may have been gifts from aunts and uncles when I was very small but I don't remember any. Geographically we lived quite a distance from them all so I'm fairly certain gift exchanges only happened when we were together for holidays, if then.
My own children are the oldest on both sides. They receive christmas gifts and occasionally birthday gifts from their aunts and uncles (the birthday gifts are from my sisters). We gift christmas presents to my nephews and nieces, but not birthday gifts (although I will send my sisters child a birthday gift, quietly, as she does send my children birthday gifts.) We also don't exchange birthday gifts among ourselves, just cards (my side) or phone calls (his side). My sisters are younger than me, and I gave them birthday gifts until they turned 18.
I feel in a way it was an easy path for us, because having kids first means when people didn't acknowledge their birthdays, I found it easy to follow their example when their own kids where born. I always gift a birth present when there is a new arrival, and after that they only get xmas gifts. I do buy them things when they are visiting though, like water guns for everyone, or a new shirt or special one on one ice creams.