I am sorry they do not send you a gift, this is really lopsided. My guess is that the wife expects her husband to take care of his mother and she takes care of her family. I do understand your situation, I get it totally. My former brother in law used to call us to thank us for gifts to his children and his kids never got on the phone or wrote thank you notes, nor did my sister in law ever call or write thank you notes. My kids were made to write them thank you notes to this same Uncle and his family, so it was very lopsided in my mind. My former MIL even mentioned once that she never got thank you notes from these children of her other son and how she appreciated the notes from my kids. Like you, we thought it was weird and uncomfortable for us. At the time of our divorce, we were sending one big family gift, like a board game, but then we got divorced and it was not my problem any longer!
My suggestion is to accept how it is and stop looking for praise or lovel for sending the gift. Cause that is the root of this, we want love back for the love we put into sending the gift and when we don't get that love back, we feel resentful.
If you want to send a gift because it makes you feel good about yourself, then consider sending one family gift. Like a board game, or a real wreathe for their front door, or 1-800 flowers.
If you feel better not sending anything, then do that. Whatever you decide to do, it will help you to change how you perceive the gift giving and accept the situation for how it is and the way it is, and do what makes you feel best.
At least your son calls you -- I have to say at this is more than I get most years. How about asking to speak to the grand kids during the call and talk to them, say merry christmas, etc. to them. Take the bull by the horns and stop waiting for them to read your mind. Men often think people automatically ask for what they need, what fills their needs, while women often expect others to fill our needs by osmosis, wishful thinking. So men think if we don't ask for whatever, we don't need it or want it. We have to speak up for what we need and want most of the time.