Well, honestly, I wouldn't have started this thread if it had been a larger amount. And, in case I haven't been clear about it, the actual dollar amount is irrelevant. It's the amount relative to what I see this sibling spending in day-to-day regular life, or relative to more run-of-the-mill things like birthday gifts, that I find so puzzling.
Ultimately, yes, of course I'll have to just let it go. I do have a very good relationship with this sibling, and I know the dollar amount of this gift is not indicative of the value of that relationship. I can't help but wonder if perhaps there was some sort of argument or disagreement between sibling and spouse that led to the meagerness of this gift. Given everything else I know to be true about our relationship and the positive, exuberant feelings that have been expressed about this marriage, it's hard for me to believe that sibling genuinely would have felt this gift to be generous and thoughtful enough for the occasion. Sibling is well known within the larger family as a "gift-giver".