If one child had a chronic illness that meant there was regular gifts that only went to them I would intervene.
But here I think that while it might have been nice, and definitely easier for you, if grandmother had sent something for both children, it seems reasonable to say to the older child that x parcel is due to injury/illness and that because he is equally loved if ever he has a similar situation the same would come for him.
I'd also make it clear to younger child that his accident has affected his sibling's life too and that it would be appropriate to offer to share enjoyment. Remind him that he would want that if other child received a box.
Both children need to learn to cope with the other receiving different treatment. They do not have identical needs and should learn to recognise this as a fact rather than constantly comparing.
I get it. Anxiety can present as jealousy. As a conviction that the world should be fair and just and intense anger when it isn't. And it really is exhausting to live with. You aren't failing because you don't know how to handle the current mess.
If, on the other hand, someone else could send them a shared present of something to do together due to being cooped up more than usual as x has sore arm that would be nice.
I would also try not to drop all outings child with broken arm can't fully enjoy. That isn't fair to big. Younger needs to learn to sit at park and colour in while older plays basketball (or whatever).