@jackiecline So I'll go ahead and put this out there because most of the comments seem to be decrying this entirely. Don't downvote me, bro!
I don't think you're being a grumpy dad, and I do think you have a point, however, here's my experience -
It's my daughter's birthday today too! She's 3. My son is 4. We have gotten the other child a small gift at each of the others birthdays. Nothing elaborate. We let them know it's the other kids special day but we don't want them to feel completely left out.
As far as raising a self centered or spoiled kid needing it to be about them, I think what you do the other 364 days of the year is going to have the greater impact.
For example, this morning, my son opened up his little present (what your in-law is calling the "not your birthday" present). It's a metal slinky. Nothing spectacular. But the first thing my son did after opening it was give it to his sister and let her play with it first because it was her birthday, not his.
With that said, I completely agree that turning it into some elaborate thing could be over the top, but to the extent that it gets everyone excited to celebrate for the one whose birthday it is, I don't see it as
inherently bad.
But you've also got to pick and choose your battles, I've found. If this is your hill to die on, then die away man. If you're saving your "dad says no" capital for like, no ear piercings before X, or no trampolines (death traps!) Or no drum kits, save it for that.