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

Parents can do what they want, if the child needs a car to get to their job why not get them one if you can afford to? Not a hugely expensive car but a good roadworthy one. If they want better then they can buy their own.

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

Surely the stepdaughter has her own grandparents /extended family?

I hate this narrative we must all just accept step relatives and treat them the same as blood relatives - they simply aren't the same and people can't be forced to feel the same for them.

@SheLovesThat It is very unusual, strange, and weird for mummy and daddy to buy a car for their child. It infantilises young adults and makes them look like babies.

The normal thing that normal teens and young adults do is use their part time jobs to save up for a car.

This.
the entitlement is outrageous. Perhaps the step daughter’s other extended family on her mother’s side could buy her a car. It’s not for you to fund this. What a cheek.

Step daughter has another parent who should step up for at least half.

Normal people, people who raise children in a blended family like they are children in a blended family, will raise kids who don't feel any of the dramatic nonsense you have typed.
None of the frantic, all children are the exact same bullshit. They aren't.
The stepdaughter has her own family on her mother's side. Why on earth are you talking about her father's side? He lives with the op's sister!

If she has been raised in an emotionally stable household,she will not have a problem with this.

It is always entitled adults that cause problems, not the kids.

Why would you spent 14k on a child that can vanish overnight?

You haven't answered if your Mum is wealthy. Seeing as she could afford private school for two, she is being mean. You and her are two seperate people. I can see why your sister is upset at your Mum. You haven't answered if she calls your Mum, Nan. Your Mum needs to talk to your sister. But that's nothing to do with you. You seem to see you and your Mum as one.

Exactly! I'm usually on the side of making sure blended children feel loved and included, but your sister is expecting too much here @SheLovesThat .

The onus was on the sister to ask, rather than to assume that OP and her mother would do the same for her stepchild.

Why would her nephews and/or OP be involved in that at all? That’s for her parents and possibly also her stepmother to figure out.

But you're not thinking about the girl. For over 10 years she's been thinking that you ARE treating her the same, because you said yourself that you've been keeping the university fees secret from her. So it must have come as an awful blow when she compared what she got from you compared to what her step-brothers got. With no warning. I feel sorry for her.

This child has been in your sister's life since the girl was 7. That's 11 years. I can see why your sister feels they should be treated the same. But, you said the girl's mum earns a lot more than your sister so, in this case, I think it's up to her mum to buy her a car. If she had no other family, I think it would be awful not to treat her the same as your nephews but here, she's surely going to be treated to something from her mum? (Maybe not a car. Maybe there's a reason her mum hasn't offered to buy her one? If it's become a problem, it's really something for your sister's husband to discuss with her ex. She should back right out of it.)

Hindsight says you should have told your sister you weren't in a positive to do the same for her stepdaughter long before your sister and the young woman might have been planning on you doing the same for her.

Does the 18 year old need a car at the minute? If she's say at college with good public transport she maybe doesn't need one right now. (Or the ongoing maintenance/petrol/insurance costs) Are your nephews going to end up helping her out financially to buy one? Can you offer her a loan?

I think you've been really mean and considering this girl has been a part of your family for most of her life I can't imagine how shit this has made her feel. Talk about making it clear she's an outsider.
Does her father/grandparents contribute to her living expenses equally? I'd understand more if they did but if you are her sole family that's awful.
Either way, £100 is extremely tight. I'm not surprised your sister is upset. You should have considered her right from the beginning when you did this to the first sibling.
I bet Christmas and birthdays are awful for her too.

Tbh I don’t think you’re being unreasonable. The stepdaughter also has her mum’s family.

But why, at any point over the last 11 years, did you not have a conversation with your sister to say that you weren't saving for her step-daughter and that she wouldn't be getting large presents from you? If you've been attempting to make the girl think she's been getting equal-value presents all this time, it should have been made clear,at least to your sister, that this wasn't actually the case! I don't disagree with not spending thousands on her, but I do disagree with not making it clear from the start.

Relevance?

Do the step daughters grandparents give there step grand children exactly the same as she gets?

I agree with you, I woild never expect my sons step mum's parents to buy him a bloody car! Wouldn't even cross my mind even if they bought his step sister a brand new Bentley!

There's part of the family and part of the family.

The girl sees a lot less of OP and OP 's mother, than OP 's nephews do. She is presumably treated well when she's with OP and her step-GM, and joins in family events when they're during her dad's time. But a relationship like this is really unlike the relationship between a grandparent and grandchild. Expecting a grandmother to spend £12k on a child that they see infrequently is entirely unreasonable.

OP 'sister presumably spends 50% of her time with her SD, so has formed a secure relationship with her. But the grandmother, however kind, will not have had all that time to reach a relationship that is similar to the one she has with her Grandsons who she sees more often .

Yes the GM should include the girl in family events, but she does not have to spend that amount of money on her.