Is it unreasonable to get a present for a sibling when it's their brother/sisters birthday?

Oh rubbish

A little something when sibling is young just helps smooth the day so everyone has a nice time. It can be part of helping a child learn about things in a positive manner and manage their own emotions.

I buy ‘myself’ a present on my kids’ birthdays as having them was pretty horrific and I deserve endless appreciation for it 😂

However I don’t get them gifts for each other. That feels odd but not problematic enough for me to make a fuss. I’d just chalk it up to whatever and move on.

I always did this - although it seems to be very frowned upon on Mumsnet! My two were always very close and good at sharing, so it was just a little extra “birthday fun” as far as I was concerned. Everyone knew whose birthday it was and the focus was always on the birthday child entirely. I actually remember my older one seeing something her sister would like in a shop and saying “but Mummy, you could get that for her for my birthday”, which I found rather amusing. It naturally fizzled out as they got older and they’re hugely supportive of each other’s successes, so I can’t say I think it’s done anyone any harm.

or you could just teach them that there’s nothing to be jealous about as everyone has their own birthday so no one is missing out 🙄

and sorry but there would be no grabbing of siblings presents anyway, who would allow a child to do that? Only parents of entitled children

No way. All this ‘Everyone wins, all shall have prizes’ stuff is behind much of the trouble with toddler adults now, in my book!
Kids (and their grandmothers) must learn that we do not need to share the spotlight with our siblings when it’s their big day, just as we do not grow up and propose to our beloved in the middle of that sibling’s wedding.
Bring children up with the instinct for selflessness in such occasions, not a feeling of begrudgery if someone else is receiving gifts/ praise etc.

A little something can help when young with jealousy. It doesn't have to go on for ever.

Stops them grabbing siblings presents, tears and stress.

We have DC with birthdays very close and so there is some element of sharing presents/present opening because far away relatives send stuff jointly. However in principle I am with your DH on this and even agree on not that many physical gifts - DC end up with so much stuff!

Personally I really don't like it when children get presents on their siblings birthday. From what I have seen growing up (and this is just my experience, so pleased don't bash me) they become entitled and really bratty.

A few aunts and uncles tried to do this with my kids and I said thanks but no thanks.

Exactly this.

My MIL always buys a little gift for the non birthday DC. My DM has never done this.

It's really not something I could get worked up about, or would want to police.

I don't think it's a common thing, or necessary, but at the same time I wouldn't object to it. It's only one gift from grandparents. The birthday boy will presumably still get more presents and more fuss made of them than the sibling.

My MIL has always done this. It wasn’t something I particularly approved of but it simply wasn’t worth causing any angst over it. It’s always something small and token. This year was some hair bobbles and a new hairbrush. She is a kind and generous person. I can be bothered to hurt her feelings or start an argument.
Unless she’s buying him a new PlayStation on his sisters birthday I would let it slide.

When my children were young I used to buy the 'not birthday' sibling a little gift, something small that cost less than £10! It was a little gesture and they still understood it wasn't their birthday.
I'm not sure why your MIL wants to do this so desperately though, especially after being told no? That is the weird part to me.

Hard no from me.

If she wants to get a gift for both let her.

Say thank you

Honestly bigger things to argue about..

You are not asking her to do this, she wants to. The baby won't know and in future years the kids will get double presents to enjoy :)