Yes Yama it's true that if your inlaws "were resentful of this (they could) cease to get (you) a gift" but I know that I would be unhappy doing that just to my SIL and not to other family members. For example, I can't imagine acknowledging the birthdays of my brother, my nephew and my nieces, but not acknowledging the birthday of my SIL. We do see them quite regularly, and that would feel rather odd (to me, at least), as if I was valuing her less than the rest of her family.
I see your point Kundry but indications over the years suggest that, yes, my SIL does want to encourage the card and gift thing. Not just she, but lots of family members on both sides, which is what I meant when I said it is the norm to give cards and a small gift in both my brother and my SIL's extended families.
Over the years, at the suggestion of another family member, the gifts exchanged between adults in the family have become small, a book by a favourite author, some incense, a candle, varies according to the known likes and dislikes of whichever recipient, but always small, a token acknowledging the birthday, really.
In the case of this particular SIL I am as sure as I can be (without having ever asked) that she does want to do gifts. Likes and dislikes have come up in conversation so, for example, I would know whether she already had a recent bestseller of not. We also do sometimes see various family members on their birthdays, including my SIL's, which does tend to encourage the gift thing.
Having said all of that, can I just add, that I only explained "another way of looking at it " in my post on 07-Apr-14 23:57:52, simply to show that there is another side to the situation described in the OP. I am not putting my SIL down, and would not want to call her on this. As I already said it is not her fault that my brother is inconsistent in remembering or bothering and, again as I already said, I can see her point, in expecting her DH to take care of sending his own cards.
I'm simply illustrating that there is another way of looking at it (but I promise you, I'd rather have my SIL than a gift
).