What a sad post to write.
Oh,not sad because your inlaws' photos in their home are larger than the ones you got. Not sad because one of your husband's siblings have never bought your kids gifts. Not sad because certain people come to parties empty-handed. Not sad because someone wants a bigger house than yours.
It's sad because for 20 years, you've been keeping score, keeping track of who bought what for whom and how certain people showed up at parties.
And it's sad because of what you're teaching your children. You write that your kids ask why their relatives don't buy them gifts, when you take great care to buy gifts for others. What you should be teaching your children is not to expect equal treatment when giving gifts. Don't teach "treat others the way you expect to, or want to be treated". Instead, teach "we give kindness always, because that's the right way to act. Sometimes we give gifts like toys or clothes because it gives us pleasure to see others enjoying something we've chosen for them. That's it." Teach your children to treat others the way the others should be treated, regardless of the payback.
It seems like for over two decades, you've been keeping track of equality, of expected treatment, and fretting over photo sizes.
You're going to be (more) miserable. Stop expecting gifts. Stop putting demands on what people should show up with when they come to a party. If you're giving a party, celebrate the birthday boy or girl, and figure out how to teach your children to be appreciative of people, not things. Stop worrying about whether your MIL buys Buckingham Castle, and instead make your home one of peace and love. Stop listening to the "mutual friend", who is incredibly NOT helpful. Teach your kids a very different lesson than you're teaching now.
To answer your questions, I'd treat your in-laws with kindness, I'd tear up the score card, I'd purge your heart of regrets, and I'd start creating a healthy and generous and kind spirit in your own children.