Do any of you do this? Buy a gift for your other kids on one kids birthday?

We do not do this and don't ever plan to. A birthday is your special day and you shouldn't have to share that with a sibling.
My inlaws got DD1 a small present on DD2's birthday last year because they didn't want her to feel left out. I think this year I'm going to make a point to ask them not to. DD1 is going to be 4 and I don't want her thinking this is a norm and expecting a small gift every time someone else celebrates a birthday!

What are your thoughts?