Are we being unreasonable not to buy my sister's stepdaughter a car and insurance

So instead of being immensely grateful for the £28000 you and your mum have spent on her sons, your sister is in a huff because you won't cough up a further 14000 for a child who isn't actually related to you?
This child has 2 parents, it's their responsibility to pay between them. Your sister saved a lot of money due to your generous gifts, she could have put some of that aside for her SD, but she didn't. That's not your fault.

Because it’s not their grand child or niece!

You can’t force people to treat your adult step children like you want them to!

Yeah, from what OP has said there isn't scope for compromise here.

No doubt the girl's mother would consider it overstepping, if OP 's mum had bought her daughter a £12k car. And that would be another Mumsnet thread

What was the will discussion? Is your sister expecting her DSD to get an equal share?

Do you have children? It’s not fair if you don’t and mum doesn’t leave you anything.

Let your sister ignore you.
Let her off.
She's an entitled CF.
What you and your mother choose to do for your adult nephews is actually none of her business.

I think you're 100% correct. A small cash gift is perfect and the expectation her blood relatives band together for a car (although it’s not the norm to gift cars)

This is wild. You’ve made a rod for your own backs here, but that kid has been part of the family for more than half her life. It’s pretty shitty to make her feel different now. It doesn’t sound like you’re all short of money and I think it’s quite mean to leave her out

Did the stepdaughter's relatives offer to buy cars or insurance for your nephews. I guess not- this blended family business usually only seems to work one way.

I say enjoy the peace. Your sister sounds like a troublemaker. If your step-niece was at your house, happily, just yesterday then that's all that really matters. Your sister needs to grow up.

Your sister is delulu.

She thinks silent treatment will get her what she wants.

Does she even think about the step-son she no longer sees I wonder.

Of course you are not being unreasonable. I find it mad that you’ve done that for your nephews but if that’s what you wanted to do then fair enough. It does not mean you and your mother have to buy your sisters SC a car. I mean what?!
mumsnet is the wrong place for this post though. Everyone goes crazy at the thought of a SC being left out. I personally don’t even think your sister should have to buy SC a car if she bought her own children one (with her own savings or something for example). You are not her parent. She already has 2. And your sister if she wants to consider her equal to her own children. Let them sort out buying her a car. Obviously it’s sad for the girl but nothing to do with you!

Why on earth does anyone get a present for passing their driving test???

I bet if each of you got a third of the inheritance, for the parents to divide amongst their own kids as they see fit, suddenly she wouldn't see her boys the same as the SD and be happy to split the money equally between them.

Anyway, she's amazingly grabby. If my mum/sister bought my oldest DD a car, I'd be extremely grateful. If they then didn't buy one for my youngest DD, I'd still be extremely grateful for the first car and tell my kids to share. I certainly wouldn't be demanding a second car from them and ignoring them if I didn't get it! What a cheek!

Do you think this is all coming from your sister, or is she trying to make her DH happy?

I hope your DSis doesn’t try pressure her some to change their inheritance.

Your brother needs to have a word with your sister.

Your sister sounds horribly grabby. She’s had incredibly generous support for her sons and is totally unreasonable to expect the same for her SD. Worse still that she is trying to dictate the terms of your DM’s will!

I agree with PP that this is probably also about her assumption for a lump sum for uni, too.

Let her continue her juvenile silent treatment for as long as she wants.

I agree. We bought both kids cars when they graduated from uni. It gives them a financial leg up, since they don't have a car payment to make every month. And we know that they have safe, reliable transportation.

It's not unusual at all.

@chirrupybird How do you ever save up enough money to buy a car on a minimum wage part time job as a teenager?

How did I do it? I worked in my job... money accumulated in my bank account... then I bought a car aged 20... how else would I do it?

@SheLovesThat It’s not unusual at all. Babies don’t drive cars, silly.

Then why treat grown adults like small babies?

All of the kids have had part time jobs from 17/18 whilst studying. My oldest child and nephews are now in full time work paying for themselves.

Why did they not buy their own cars like normal people do?

Your sister sounds greedy and entitled.