Some parents play favorites. It stinks. But it's your husband's problem with is own parents - they didn't come to the wedding, they don't accept or value you, and you are powerless to do anything about it. I'm not sure if your husband should stand up for you more, or if that's pointless because they aren't the type of people to respond.
It's also possible, on some level, that they LEARNED from skipping your wedding that it was lousy, and they vowed not to do it with the 2nd son.
But nothing you can do will make them able to attend your wedding, right? You have to let go because the person you are punishing is yourself. Anger eats us up and makes us the people we don't want to be.
You obviously feel that it's wrong to penalize your BIL for what his parents did or didn't do, so staying home is not going to help that. I mean really, why shouldn't any couple be able to talk about their fabulous event? That's what you would have wanted for yourselves, so why deprive HIM and his wife?
In fact, it may be even more powerful if you go, listen to all the stories, say how much you would have loved to be there (don't go on about cost), and just revel in the details. Smile and take the high road. When appropriate, you can look at your husband and say, "Oh remember at our wedding when X happened? What wonderful memories!" and be sure your husband is in on this and participates. Hold hands and show the inlaws and everyone else what a great match you are, how fun you both are, how much you love each other. Then say to your BIL and his wife that you are so glad they have such great stories and photos too.
You can show the others the right way to act, and not let bitterness eat you up. But you cannot go through life avoiding family functions because you are mad at 2 of the people there (unless everyone is toxic and that's the path you choose, no contact, no celebrations).
It's not exactly the same thing, but my husband's ex was always horrible to me, and to him. When their oldest daughter got married, the ex's sister through a HUGE shower (I'm talking 60 people)!! I got a token invitation and no one else in my family did (not my mother, nobody). I agonized like you are. I know the ex is a big phony and has few friends so most of the guests were friends of the bride & groom. But I went - head held high, looking as fabulous as I could muster, and was super charming and funny and interested in everyone else. Knocked everyone's socks off. If I hadn't gone, it would have confirmed what she's said about me. This way, people got to see the real me.
If you stay home, it confirms that the inlaws are right, that you aren't the type of classy person or good wife they expect. If you go and behave as you wish they would, you will be true to yourself and show them a lesson much more important than staying home would do.