To think this is batshit in the extreme?

[AintNobodyHereButUsChickens]

My son is 9 now, me and sister in law were pregnant at the same time although I was due a couple of months before her. She never said it to my face but she was furious that I'd gotten pregnant before she had, it meant that I would give the in laws their first grandchild and she wouldn't. Anyway, as I was nearing my due date she used to say stuff like "If my DD was born now she'd be the eldest instead." (Bearing in mind that she was due two months after me!) On a few occasions she actually said she wished her DD would be born early and she would hint that her consultant said he needed to do an early Csection The babies ended up being born in their intended months, mine being first.

Anyway fast forward a few years and they moved away to the opposite end of the country and we hardly see them anymore. Recently, DN came up for a visit and her and my DS got talking about their birthdays and their ages and who was the oldest etc. They got into an argument because DN was insistent that her birthday was.... three months earlier than it actually is. DN came and asked us when her birthday is (she was at our house at the time) and we told her when it was, her real birthday, we showed her on the calendar. And she refused to believe us, she kept saying "So my mum's been lying to me my whole life about when my birthday is? My birthday is not 'X' but actually 'Y'??" We got her to ask MIL when her birthday was and MIL was like "Errm, it's , why?" And DN just kept saying "So my mum's lied to me this whole time?"

What the hell?! Who does that??

So what happens with all her cards and stuff from friends and family? Does she just think they’re all really crap at getting them out on time?

Someone needs to tell her to stop being a dick.

I know someone who told her child her birthday was a few days later than it was so she wouldn't have the nagging and excitedness for long. Only worked until the child was about 7.

How have birthdays worked? Surely the whole family have seen DN on the actual birthday at one point in their life?

WTF!!?? Surely it's not possible to deceive everyone, incl the child, about their true birth date for so long? What about the school etc? And b-day cards, gifts etc from wider family? Has the poor kid been told that the school has it wrong and her wider family don't care?

And if your SIL has been doing this, she's seriously deranged. In fact a systemic lie like this verges on abuse.

Who cares who has the first grandchild! So ridiculous and petty! So glad I don’t have kids and all these dramas to contend with.

Yeah, this is batshit/bullshit but not in the way you meant!

I believe it!

My DH ex (who we still hear off via various mutual friends) had a baby the year after we did and she made a massive fuss about her babies due date being our sons birthday. Her baby was born early so 7 days before our sons birthday. She sent messages round to people (sil showed me the one she sent and posted on sm) saying that although her baby was born on x date forever in her heart his birthday would be y date (our sons birthday) and they would treat that as his r birthday too and be throwing parties on that date . Utterly batshit

Anyone with your SIL's mentality should not have children.

She is not fit to be a mother !!!

What date is on the birth certificate?

Surprised she didn't steal your chosen name too.

Presumably when she moved miles away and made new friends she could tell new friends what she liked re her birth date. So no one may be any the wiser.

School and GP is a weird one though.

I know someone who told her child her birthday was a few days later than it was so she wouldn't have the nagging and excitedness for long.

Huh? makes no sense.

Ex pil agreed and tried to talk ex h into me getting an abortion (part of the reason he became ex is that he didn’t think it was unreasonable for them to request it).

That is disgusting, what is wrong with people

Bollocks OP.
You say you lived near each other for a few years before they moved away, so presumably you celebrated DN's birthday each of those years? And then sent Birthday card/presents after they moved away?
And DN started school which required a birth certificate and frequent references to birthdays etc. (former YR teacher here)

DN is winding you up - (or you are winding us up)

Now I don't believe in woo, but bumbling along on the bus yesterday to a hospital appointment, I was listening to an old "No Such Thing As A Fish" podcast, and they mentioned ....

www.flavorwire.com/302388/robert-louis-stevenson-gave-his-birthday-away-to-a-little-girl

When he was 41 years old, Robert Louis Stevenson gave his birthday away to a young girl named Annie, the daughter of the American Land Commissioner in Samoa, who was understandably bummed out by the fact that her own birthday fell on Christmas

Which, given the OP strikes me as sufficiently intriguing ...

i ant think how this could be true.you and your mil sending cards pressies.
anual checkups at the docs.or any apt at the docs.i can often remember being young athte docs an them asking me my date of birth over a few things.
starting school.
having a birthday at that age is a big thing at school so the teachers would hear when it was her birthday and so how did she lie to them too?

I put yanbu just in case it’s true, but I can’t see how?

I needed a birth certificate to register my children in school and doctors.

So surely, the school will know her birthday. Especially in reception when they tend to put children’s pictures to their birth month.