Sorry I am late to this.
Scoop, I am glad you talked to your DH about this. This is an insult to him as much as it is an insult to you. You are important to him, so you should be worthy of respect and consideration to them. They don't have to like you, but they do need to at least acknowledge your importance to him, and they would if they truly cared for him. Have you ever thought you are the bat the beat him with? By being such P/A jerks about this, and FIL is just as responsible, they are disrespecting him. It is up to him to call them on this kind of behavior.
I have a lot of experience both as a recipient of awful gifts and as a parent giving gifts. I met my DS's GF of six months over the summer and bought her a Christmas gift in December. I sent it to DS's address and when he got it, he told me they were no longer together. I told him to keep the gift and give it to someone who would appreciate it. Even though I had met her just once, it was important to me to acknowledge this girl's importance to him with this gift. He had never been with someone so long. If he does have an SO and Christmas or some other gift giving occasion comes along, you bet I will buy something for her even if I have never met her. I am hit or miss with gifts. Sometimes I am spot on and other times I totally miss, but I give a gift and let it go. I make sure the recipient can exchange it and I am not offended if they do.
I am of two minds on this. You are right to be offended by their treatment of you as it is an obvious slight. Yet, by being offended, you give them power to hurt you. You are putting more effort into the relationship than they are just by being mad. So a large part of me says to let it go, make a joke of it, even say to DH, "You owe me $20." When they ask, say in an uncaring manor that you bet DH they would totally forget your birthday again this year. The other part of me would note this slight, and make sure to put in as much effort for their special days as they put into yours. I would make sure DH knew about it, and I would make the comparison with what you received as to what everyone else in the family received very concrete so he could not just ignore it.
Either way, putting as much effort into the relationship as they put into it is the way to go. Make gift giving for them DH's responsibility. If he tends to go overboard, put it into the budget how much he can spend on them. If he tends to forget, don't remind him. Let him do what he will for his Ps. Don't even arrange for your kids to do something for them. He doesn't do this sort of thing for your family, don't do it for his. His family, his responsibility. If your PILs were different, things would be different, but these are the consequences of their own choices.
The sort of thing your MIL is doing comes around eventually. Be patient, let it mature, then enjoy the show.