Is DH the brother and you are the "in-law", so the request is more "this would really help me out bro" vs "this is a need so it's fair"...
If it's brother/sister, I can totally see the ask - with my allergies, when my sister was hosting out of town family and my local brother was going, too, as the 2 locals, she told him he was handling dessert and she was handling dinner, and that was that - she wasn't gonna ask me or my out of town sis for anything since we bought the plane tickets, paid for hotel rooms, and then were doing an extra drive on vacations. So, he "got told" what he had to bring, which he was fine with b/c it was the cheaper, easier option (b/c he did go buy it vs trying to make it)...
OP is also bringing technically 4 adults "families" - hers and each of her kids (even if each family is a "1")...so, if this is a "4 family" request, 2lbs/shrimp each isn't a big ask, when as PP said, you can get them for $5-8/lb on sale anywhere right now...
That said, if you don't like it, just say no and skip...I do that a lot when my other siblings are now having holiday dinners 2-3 hours away where I can't eat anything - it's not worth the time and effort for me to go and then eat little, but nor would I ask them to make a 2nd complete different holiday meal
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